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2-stroke top end rebuilding & performance

1. This s the late model CR250 1992 to 2000 Unlike earlier models there is no manual engagement bolt However there s still an actuating rod in the cases and cylinder which must align propery This is a layout of the CR250 Qs to 1991 This system uses a complicated rack and pinion linkage to control the rectangular v left and nght exhaust ports The cylinder and crankcase actuating joint have to be ey itan ously with controls on the cylinder This s view of the forked actuating rod from the This s the correct position of the boltom of the cylinder Notice that it 1s cracked Thal 1988 to 1991 Honda CR250 HPP happened because the cylinder was tightened down pinion shafts when the cylinder is with the actuating rods out of alignment This part seated on the cases correctly needs to be replaced 113 TWO STROKE TOP END REBUILDING You can quickly check the operation of the HPP system of a late model CR250 1992 to 2000 wn ow 17 mm cap bolt from the left side of the cylinder The L and H marks stamped on the cylinder indigate and high pm positions of the valves The line carved in the center rod is the left HPP valve s G the engine with this cover removed to check the valves but don t let it gel exposed to dirt This s a layout of the CR250 1992 to 2000 HPP system The center valve pivots from one end and a rod through the front center of the valve links the two sub exhaust vaives The center valve
2. time area for an rpm peak of 7 000 The rear transfer ports need to be reangled to oppose each other rather than pointing forward to the exhaust port This changes the flow parcern of che transfer ports to improve sca enging efficiency from 2 000 to 5 000 rpm For both of these types of cylinder porting changes to be effective peh engine components need to be chdaged sean Cylinder head sh affects the powerband cay speaking a cylinder head O p small diameter nbu k er and a wide squish band cn with a high compression ratio is suittd for low end and midrange power A cylinder head with a wide CYLINDER HEAD MOO shallow chamber and a narrow squish band and a lower compression ratio is suited for high rpm power Cylinder heads with wide squish bands and high compression ratios will generate high turbulence in the combus tion chamber This turbulence is cermed maximum squish velocity MSV and is 136 O rated in mecers per Yecond m s A ed for supercross cylinder head should have A V rating of 35 m s whereas designed for morocross sold RA an MSV rating of 25 mis The ay to accurately determine the ating of a head is by measuring S basic engine dimensions and inputting the numbers inn a TSR compurer program called SQUISH In the model tuning cips chapters the SQUISH program was used 1o calculate the modified head dirnensions Aftermarket companies such as Cool Head also offer cylin
3. dimension between a crank half and the rod Then press the crank together and prepare f f error aligning which requires a jig with live centers and a dial indicator pressure to assemble the crank He then uses the square to align the scribe lines on each Alyweight so the crank is assembled very close to truc 4 As the crank is pressed together the trued Thi Goromplished by hitting the w N of the Ayweights with a largetGyass hammer and chen wedging i DyWeights apart ac the narrow side With a large tapered chisel and technician uses a feeler gauge oh ammer Ir sounds very caveman like monitor the clearance becween th connecting rod and the cru washer The proper clearance med in the service manual eo al also lists an overall crap Wealth spec chaceanbe nee caliper Truing the Cranky C L During shy tine process heredi nician sdports che crank berween live centers Knife edged bearings must be used if the machined centers of the crank ends are damaged Dial indicarors are used to measure the runout of each crank end and the side runout of the flyweights 3 If the crank has side runout the flyweights aren t parallel and must be adjusted before the crank ends can be bur this is the way the pros do it OF course the crank must he removed frorn the jig before the runout can be adjusted otherwise the dial indicators and jig centers will be damaged The crank ends have runout because th
4. on the crankcases Here ate some tips for installing the cylinder and engaging the HPP drive mechanism 1 Make sure the teed valve is removed from the cylinder CR250s have such large intake ports that the rings tend to slip out of the ring grooves during installation of the cylinder This cakes rhe spring pressure off the cam spindle Tuen the engagement bole 1 4 tuen clockwise You should fec it positively lock into a groove and scop Remember that che HPP engagement bolt is a spring loaded derent not a threaded bolt Slide the cylinder down onto the piston and rings use a screwdriver to push the rings back in che grooves until the rings clear che intake port EXHAUST VALVE TIPS AND TUNING 2 The HPP mechanism should be engaged while the cylinder is being installed just co keep the cam spindle in position The cylinder will stop about 3 mm from the crankcases because the cam spindle and the rack are misaligned Disengage the HPP mechanism by turning the engage ment bolt 1 4 mtn counterclockwise Grasp the right side valve lever and wiggle it the cylinder should chen drop evenly onto the crankcases 3 Bolt che cylinder down tight The best way to engage the HPP mecha nism is to insert a screwdriver in che right side pinion shaft and turn it counterclockwise Now tuen the engagement bole clockwise You should feel the engagement bolt lock positively in position If you try to rotate it too far you will bend the c
5. position 6 Drop the drum valves onto rhe rack so the valves are in the full open position Don r pay attention co alignment dots or marks on the valve or rack just remember char the valves should be open when the rack is pulled out and closed when the rack is pushed in 1992 1997 KX125 and 1993 2004 KX250 KIPS Timing Procedure The system on the KX125 and KX250 uses both wedge and drum valves wich racks This is the best exhaust valve system for performance bur the most difficult co maintain Here are some tips for retiming this KIPS system 1 Install rhe wedge valves in the cylinder and the actuating rod and lever Squirt some pre mix oi on che parts 2 Pull the wedge valve into the Fus open position place the gear on end of the rod and rotace th counterclockwise until che against che stop plate Th on the rod and cighipf Ni clockwise ir is a lel 3 Place the dru s inro their respective akeni the rop of the gears ith the cylinder ush che lower rack into base plac amp ghd Dolt the seal pack on the rack int the cylinder 4 Pull che rack out until it stops and push ir in 1 mm now it is in the correct position to install the drum valve Before you push rhe drum valves down make sure the wedge valve and drum valves are in the full open S position Push the drum valve down with the two gears firs it must engage che v 118 The most common problem with top nght side of the cylinder ti s
6. priced lower chan OEM phyons If the cylinder bore is sli 0 005 in with onl ut orn up to Q Snail arca of bare aluminum ex Yeu can install a Pro X o A Piston The Pro X sions we el cas Seal increments than Wiseco pistons but a wider range than the OEM pistons For example Wiseco pistons are sized in 0 010 in increments and Pro X pistons are sized in 0 001 in Before attempting to order a Pro X piston you must measure the cylinder hore at the smallest point and allow 0 002 in clearance between the piston and cylinder increments 104 Wiseco pistons are ideal for overbaring and electroplating The Wiseco rings are compatible with all types of nickel coati X MEASURING THE RING GAP Measure the ring end gap to determin if che rings are worn Place thesin the cylinder and use the pistoi x it down about 1 2 in froy top evenly spaced Now use Ateghoefauge to measure rhe width Normally the masttun gap is 0 018 to 0 025 in CYLINDI EXHAUST VALVE NING TIPS DoessfouPcylinder have burnt on mud ny ue heavy brown oil glazing Q cylinder bore or gooey oil on the haust valves If so there is a way 10 dean those pares without flammable cleaners Go to the grocery store and geta can of aerosol oven cleaner This stuff is grear for cleaning the carbon from the exhaust valves withouc completely disas Boyesen Precision Porting makes trick circlip instal lation tools for 125 and 250 cc dirt bikes T
7. rare their pipes bur the pipes are still different dimensions That s why you can buy two of the same model pipes from one manufacturer and they will perform slightly differently TUNING GUIDE TO PERFORMANCE MODIFICATIONS Whats a Rocket Engine Doing in the Pipe When FMF introduced its SST pipe the manufacturer described it as a rocker engine in che pipe Actually it s a rapid convergence cone and it is shaped like the business end of a rocker engine By constricting the outler of the stinger before the silencer it serves co raise che pressure in the pipe and the piston ctown temperature It has the greatest effect ac high cpm In shifter kart racing RCE markets a pipe with a selection of tuning cones thar feature different angles and diameters co adjust the tuning effect for different rpm ranges The opposite of a rapid convergence cone is a rapid diffuser A bulge in the junction becween the convergence cone and che stinger was used on the Yamaha IT and YZ490 models of the mid 1980s That shape prevents a backup of pressure in the stinger mainly because those old bikes had a sharp bend in the pipe co compensate for FMF uses a rapid diffuser in its four stroke header pipes and calls ic a Power Bomb A multi phase tesonator is commonly used on snowmobiles and it works differently on a two stroke engine A rapid diffuser allows for less Joss of energy when waves moving in opposite direc tions pass through each ot
8. sufficient co run with stock or after market exhaust systems CYLINDER HEAD After overboring the cylinder che head s dimensions must be changed ro suit the larger piston First the head s bore must be enlarged to the finished bore size Then the squish band deck height must be set co the proper installed squish clearance The larger bore size will increase the squish turbulence so the head s squish band may have to be narrowed I he volume of the head must he increased to suit the change in cylinder displacement Otherwise R engine will run flat at high rpm ot eg in the midrange from pnrt EXHAUST VALVES Kia When the bore size is sl the exhaust valve co pist be checked and adj This pertains to the types aust valves that operate wi seproximiry of the piston Ce valves aren t modi fied ch amp iscon could strike the valves and cause serious engine damage The normal clearance between the exhaust valves aod the piston should be ar least 0 030 in or 0 75 mm CARBURETOR The larger the ratio between the piston s diameter and che carb s size the higher the intake velocity Overbored cylinders produce higher intake velocity which 106 draws more fuel through the carb OF course a larger engine wil necd mote fuel Normally when you overhore an engine 15 to 20 percent che slow jet will need to be richened and the main jer will need ro be leaned Start with the stock jetting and make adjustments a
9. Modern bikes use external than internal rotor flywh els so have greater Mywheel inertia A makes an internal rotor flywheel gives quicker rhrorele respons Sa IGNITION TIMING The ignition cim Qa a minimal effect on the powdkad Retarding the timing has th F reducing the hit of the p hen in the midrange and extending he top end over rev Over rev is a sing term thar describes the useable length of the powerband ac high rpm The scientific reason for che shift of the powerband to extremely high rpm is that the remperature in the pipe with che retarded timing because the burn cycle cakes about 55 degrees of crankshafi roration When increases the timing is retarded the burn cy 142 rotor Q and conrinues into the pipe flywheels They bave a larger diame ne fog the exhaust gas temperature y Ye ises the velocity of the waves making them mote synchronous with the piston speed and pore timing of the cylinder Advancing the timing increases the midrange hit of the powerband but makes the power flarcen out at high rpm The tclatively long spark lead time enables for a greater pressure rise in the cylinder before ches TDC l his produces more torque in the midrange but the high the piston tea pressure contributes to pumping losses at extremely high rpm ENGINE MANAGE SYSTEMS Motorcycle engines have always been at the forefront of engine design The next leap will come in
10. This desig Yamaha to position the v deine tronic ORe varying the exh rt Unfortunately l 2 timing iminum tab on the valy worn allowing che valve to rotatedarcher in the fully closed position Eventually after abour three years use the tab wears enough char the valve strikes the piscon causing damage to the piston Yamaha s exhaust valve is cheap to replace 1 recommend replacing the valve when che tab wears more than 0 030 in 0 7 mm In 1994 Yamaha changed the engine design of the Y7125 and included the next generation of exhaust valves This ves positioned at a 45 degree angle over the exhaust port The system is similar ro the one employed by Suzuki Yamaha experimented with resonator cavity volume and vents for pressure bleed off and Overall this is a very reliable system temperature control Occasionally the pins thar fit through the ends of the valve to interface with the actuator lever vibrate out causing the valve to strike the piston Those pins are a press fic but you can add some Loctite Instane Adhesive to the pins for added protection Yamaha has added springs to the valves to control high rpm valve flutter but include a positive seal between the valve future innovations could and the cylinders valve pocket In 1999 Yamaha redesigned the 50 and engine exhaust valve iston crown causing catastrophic engine damage Whenever I service a KTM cylinder system The model
11. and tbe amount of dirt or other debris in the intake air Bikes that are run hard with dirty air filters may wear our pistons in only six hours while bikes thar are ridden easy with clean filters and adequate fuel octane may last 60 hours This Motion Pro piston pin extractor saves you from hammering on the rod to remave the piston MEASURING THE PISTON Lr is bese to measure the piston with a caliper Digital calipers cost about 100 at industrial rool companies such as Enco or Harbor Freight A digital caliper is casy to use and gives accurate measurements on the piston diameter and cylinder bore Measure the widths of che piston front ro back just above the intake cucaway this is che widest point of the piston Check the maximum wear specs in your service manual Check the piston for derona rion marks in the crown cracks in the skire or seizure marks Look at the underside of the piston crown for a large black spot The spor is burne oil deposits chat adhered co the piston because the piston crown temperature was too hot This is an indication that the carb main jet needs to be ticher Ko LETTER DESIGNATIONS ON O CYLINDERS AND PISTONS The Japanese manufacturers pajletscr designacion sysrem for pl They intend for you to pistons based on che cylinder In u cant guarantee chat all pares wil actly the same size The size variance if based on an acceptable level of quality Tool bits become dull t
12. che frame from the cop shock mount Irs important to check for warpage of the head every time you rebuild the top end S 6 Shasteted Skirt irts of this piston shattered because ARE pisron ro cylinder clearance was too rattle in the cylinder bore it develops stress cracks and eventually shatters a When the piston is allowed to 7 Snapped Rod The connecting rod of this engine snapped in half because the clearance between the rod and the thrust washers of the big end was coo great When the big end bearing wears out the radial deflection of the rod becomes excessive and che rod suffers from corsion vibra tion This leads ro gonnecring rod breakage and catastro egine damage The big end clear uld be checked every time yi 1 che top end To check the K D clearance of the S gt inserr a feeler gauge rod and a thrust washer berw NY Jech maximum wear limits in your SA connecti factory service manual 8 Four Corner Seizure This piston has verrical seizure marks ar four equally spaced points around the circumference A four corner seizure occurs when che piston expands faster than the cylinder and the clearance between the piston and cylinder is reduced Also common a single point seizure on the center of tbe exhaust side of the piston occurs only on cylin ders with bridged exhaust ports The main causes of single poine seizure are too quick warm up carb jetting main jet or too ho
13. diameter or sleeved down carb 26 mm cc 34 mm for 125 cc 36 mm 250 cc AND HIGH RPM Larger carb 28 mm for 125 cc 39 5 mm for 250 cc MIDRAN for 80 COMPONENT Pipe oO LOW TO MIDRANGE Fatty orn gt MIDRANGE AND HIGH nnn bese or rpm COMPONENT sure Ke Arrestor LOW TO aes Short small diameter MIDRANGE IGH RPM Long large diameter ROWENT Ignition Timing or Advance Timing O MIDRANGE Stock timing MIDRANGE AND HIGH RPM Retard timing COMPONENT Flywheel LOW TO MIDRANGE Add weight COMPROMISE Stock flywheel MIDRANGE AND HIGH RPM PVL internal flywheel COMPONENT Fuel LOW TO MIDRANGE Super unleaded 93 octane MIDRANGE AND HIGH RPM Racing fuel 105 octane for high rpm High rpm powerbands usually require frequent use of the clucch to get the engine up into the rev range where che powerband is mosr effective An engine that sustains high rpm requires more frequent replacement of parts such as piston and rings reeds crankshaft bearings and cluech plates Also the carb 134 jetting becomes more critical IF the main jet is one size too lean rhe piston can seize High rpm powerbands have high compression ratios and fuel selection is critical Mosc tuners recommend racing fuel because the specific gravity of these fuels docsnit vary with the season like super unleaded pump fuel TUNING GUIDE TO PERFORMANCE MODIFICATIONS Before you begin modifications you need ro decide what you
14. horsepower and rpm the narrower the powerband Conversely the lower the power peak the wider the powerband Somerimes you can ger the bese of both worlds Exhaust valves systems have made the biggest difference in widening the powerband Innovations in exhaust pipe design and ignition systems have also contributed to making two stroke engines as tractable as four stroke engines Riders should choose a powerband according to their skill levels terrain obstacles and maintenance practices Here are some tips on how ons the right powerband lt SKILL LEVEL Generally spe eginning riders need low ty midrange powerbands while expg gin can benefit from top tbands here ace excep Z Supercross bikes have low to midrange powerbands because he steep fat spaced jumps are posi Hned so close co the curns Keep in mind chat low end ro midrange power bands are cypically easier to use In conditions where traction is minimal and the terrain is particularly technical low end power will allow you to keep the bike under contol and ultimately go faster Also torquey bikes are more fun ro ride casually Expert and rop level riders need high end power ro be competitive bur the extra juice can slow down lesser riders even in good condi tions and is a handicap in slippery difficult conditions TERRAIN OBSTACLES This term describes a variety of things ranging from the soil content to eleva tion cha
15. into the engine Use a stiff plastic brush and hot soapy warer to clean off the grit and grime around the base of the cylinder on the carburetor and intake boot and especially underneath the rop frame rail Degreaser can be used on it on rubber or gasker surfaces meral surfaces but take care not to lt lt TOOLS You ll need at least some 3 8 metric sockers and wath open end wrenches a rhe edges on the cylin head nuts and shouldn t ay rop end rebuilding ose pliers for removing ci a gasker cool to scrape EA gaskets away For soft tools 8some shop cowels aerosol oven cleaner a Scorch Brire pad a locking agent such as Loctite a gasket scraper a brush and a bucker of soapy water You ll also need a compression tester a feeler gauge and a digiral vernier caliper COMPRESSION TESTING A compression tester is a useful diag nostic cool readily available from Sears 100 CHAPTER SIX or auto parts stores Buy the threaded type and make sure the kit comes with an adapter thac matches the spark plug threads of your engine Performing a compression tesr is simple Start by removing the spark plug threading in rhe adaprer and holding the throttle wide open with the kill burton on This prevents any spark and enabl the engine to draw in maximum ai Then kick start the engine sevi until rhe needle on the pr peaks The pressure readin two main factors the and the altitude ac The engine is cesred The c
16. knob too much or in the apposite direction coupthcclopkwise D Starting in 2001 Suzuki changed its exhaust valve system every year until 2004 on the RM250 fiddling around with everything from electronic solenoids to cam levers and other components Finally n 2003 Suzuki settled on a copy of Yamaha that sidestepped its patent My advice is If the valves move through their operation don t attempt to disassemble and clean them 119 This is the onginal power valve design that Yamaha patented in 1972 The system is used on 2125s from Kros and 2 WR250s from 1981 to 1998 In the early on the left side of the cylinder and positioned the valve e eventually contacts the piston causing a total top end You lose a little power but save 500 or more in the long run should be parallel when coupled co the lever and tod If the spring cabs are crisscrossed che valve t will be limited and won t open fully In 1996 Suzuki redesigned the RM250 engine going back to a design reminiscent of the 1987 model RM250 For this model Suzuki modified the Honda HPP design used on the model CR250 However a A on 8 plagued tbis system Instead of the center valve Suzuki in a passageway of the added mechanical fr A made rhe tion half open Kas the engine co run Mac TAEMA pof che valve was also a p KA The leading edge thar faced Ki was too square and sharp full apen positi
17. most common air leak at the magnero side crankshaft seal roo lean carb jetting too fat advanced ignition timing or faulty igniter box coo hot spark plug range too high compression ratio or coo low octane fuel 13 TWO STROKE TOP END REBUILDING 13 Blowky This piston didn t fail in operation but it does show the most common problem blowby The rings were worn past the maximum end gap specifica tion allowing combustion pressure to scep past the rings and down the piston skire causing a distinct carbon pattern Trs possible that che cylinder wall s crossharched honing pattern is partly co blame If the cylinder walls are glazed or wom too far even new rings won t seal properly ro prevent a blowby problem Flex Hones are available at most auto parts scores They can remove oil glazing and restore crosshatch honing marks thar enable che rings to wear co the cylinder and form a good seal If you purchase a Flex Hone for your cylinder the proper grir is 240 and the size should be 10 percent smaller than the bore diameter 4S Scattered This piste was allowed ro run way past its service life Too much clearance beeween che piston and cylinder wall caused stress cracks to form at several 14 Shay paints Amazingly enough this piston shattered and the engine quit withour causing any significant damage Most of the debris was deposited in rhe exhaust pipe I flushed the crankcase replaced the crank
18. piston the Wiseco GP uses a nickel crown coating that reduces detona tion a moly coating on the skin that is a dry film lubricant lightening cutouts inside the piston for wider exhaust porting and 50 percent of its mass is centered in the piston crown for better thermal efficiency The ning is pinned on center to allow for wider transfer ports TUNING GUIDE TO PERFORMANCE MODIFICATIONS than the stroke Conversely the popular MX configuration is termed under square or long stroke Long connecting rods are commonly thought to produce more leverage bur the real advantage of a high rewing engine is that the piston dwells longer ac TDC and allows for a greater pressure rise and hopefully more brake mean effec tive pressure BMEP or the average pressure per square inch in the cylinder from TDC to BDC The manufacturers fiddle around with bore stroke and rod combos all the rime The latest rage for the amateur class for 125s where the 80 over rule applies is a 55 5 mm bore and a 55 2 mm stroke with a 109 mm rod Suzuki has determined that co be the winning combo and requires its support riders to have their engines modified co thar spec For the Kawasaki KX80 the magic numbers ate 48 5x53x92 Kudos to Pro Circuit for doing the legwork on thar one So how does one find that magic combo You could spend loads of time and money to try every possible combi nation or yoo could use a simulation program such as Vi
19. response The otber circuit has a shallow advance curve wit a high speed retard circuit harg the spark close ro TDC Th cy d the top end or hole sl iech Essentially combustios abour 55 degrees of crank to occur Retarding che spar es more heat from combustion to shift into the pipe and our of the cylinder Raising the temperature in the pipe taises the tpm peak in accordance wich piston speed FLYWHEEL WEIGHTS A heavier flywheel will smooth out power delivery The ftywheel is weighted to improve the engine s tragcability at low to mid rpm Flywheg Nyeighes are best for powerful bios decent lov end and an explyfiv G The weight smooths our the Aje age reduces wheel spin which wijt improve che drive out of comers O jst misconception of Alywh ea s is that they increase bor If an engine doesn t have enough fw end torque in the first place will actually be worse with the extra heel weight Steahly manufacrutes thread on flywheel weights thar thread on to the fine left hand threads on che center hub of most Japanese magneto rotors Thread on flywheel weights can only be used if the threads on the flywheel are in perfect condition This flywheel has been modified for a longer stroke length The stock big end pin is bored off center and a steel slug is welded in place and bored to size 141 TWO STROKE ENGINES A centered spark plug head ts threaded to a Goodson Automotive
20. source of a mechan ical or runing probleou This iva gulde fo the most common mecbanical pbk 1 Perfect Brown Crown The crown of this piston shows an ideal carbon pattern The transfer ports of this two stroke engine are flowing equally and the color of the carbon pattern is chocolate brown That indicarcs that this engine s carb is jetted correctly 2 Black Spot Hot The underside of this piston has a black spor The black spor is a carbon deposit chat resulted from pre mix oil burning onto the piston because the piston s crown was too hor The main reasons for this problem are overheating due to lean carb jecting or coolant system failure aS 3 Ash Trash This piston crown has an ash color which shows thar the engine has run hot The ash color is actually piston material thar has stacced to flash melt and tutn to tiny flakes If this engine had been tun any longer it probably would ve devel oped a hot spot and hole near the exhaust side and failed The main causes of this problem are co lean carb jercing too hor spark plug range coo far advanced ignition dming too much compression for the fuel s octane or a general over heating problem 107 TWO STROKE TOP END REBUILDING 4 Smashed Debris This piston crown was damaged when debris entered the combustion chamber and was crushed berween the piston and the cylinder head This engine had a corresponding damage pattern on the head s
21. squish band The common causes of this problem are broken needle bear ings from the small or big end bearings of che connecting cod broken ring ends or a dislodged ring centering pin When a problem like this occurs it s important to locare where rhe debris originared Also che crankcases must be flushed out to leftover debris that damage again If che debris originated from the big end of rhe connecting rod the crankshaft should be replaced along remove any could cause the same with the main bearings and seals 5 Chipped Crown Drown This piston crown ch at the cop ring groove beca head gasket leak The coola i drawn into the combustion Be on che down stroke gPth pisron When che coolant hits ch amp Piston crown it makes the aluminum bottle and eventually cracks In extreme cases the head gasket leak can cause erosion ac the top edge of the cylinder and the corcesponding area of the head Minor leaks of che gasket or O ting appear as black spots across the gasket surface When an engine suffers from coolant being pressurized and forced out of the radiaror cap s vent tube it is a strong indication of a head 108 gasket leak In most cases the top of the cylinder and the face of the cylinder head must be resurfaced when a leak occurs Most MX bikes have head stays mounting the head ro the frame Over time the head can warp neac the head stay mounting tab because of the forces transferred chrough
22. strong throtde response to clear obstacles tion Js there an ideal length or is longer better For a typical 54 5 mm stroke the rod can range from 102 to 12 mm Why use a spacer plate and is there an alternative A spacer plate is like a thick base gasker usually made of aluminum with a standard gasket on each side to provide some sealing means The spacer plate serves to compensate for che difference in the extra rod length In order to use a spacer plate the cylinder base studs and the power valve linkage rod must be longer The head stay brackets must also be slorted and tbe exhaust system brackers will need to be repositioned The use of spacer plates enables che port timing ro remain close to stock which will help the midrange to top end power The alternative to a spacer plate is to machine the cylinder head s squish band and combustion chamber to accommodate the piston travel This would also serve to retard the port ciming due to the relative position of the piston TUNING GUIDE TO PERFORMANCE MODIFICATIONS Here is a comparison between a V Force left and conventional stock reed cage right The V Force has do the reed area bul also double the angle V Force reed valves generally improve the low end to midrange CARBURETOR MODIFICATION Lo gencral a small diameter carburetor will provide high air mass velocity and good flow characteristics for a low to mid tpm powerband A large diameter c
23. tends to get caked with carbon hindering its operation Clean the valve with a wire brush and pay special attention to the center of the valve that controis the full open and full closed position of the side sub exhaust valves You can use a small diameter round file to remove the carbon from the center valve 114 HPP Timing Procedure Use the following procedure to time the HPP system 1 Install the HPP valves and levers and tighten che pivot nuts Place the washer on the stud first then the lever marked left and right and then the flanged center hushing wich the flange side facing p 2 Turn the cylinder positon shear to the left de down To rectly slide it tops then move it right 2 nfn gt Rotare che rack so che sqyare notch faces you Now the in the correct position to stall Yhe pinion shafts Carefully itp the cylinder right side up y thouc changing the position of the rack 3 Close the valves and install the left pinion shaft with che screwdriver slot facing che 1 o clock position Install the right pinion shaft with the screw driver slot facing the 11 o clock position sce photo for correct posi tions A simple way to determine if the pinions are mistimed to the rack is co look at rhe screwdriver slots The wrong position is with borh slots facing 12 o clock Installing the Cylinder and Engaging the HPP Drive After timing the HPP mechanism the cylinder is ready ro be installed
24. to 0 980 Neng on the dlisplacemen ic engine The larger the displgtement the greater the clear 17 Arrow Forward This piston was installed backward meaning thar the arrow on the piston crown was pointing to the rear of the bike The ring end gap was aligned wich the exhaust port and the rings expanded out of the groove into the exhaust port causing them to shear off in the exhaust port Tbis engine suffered catastrophic damage and required a new cylinder and piston TWO STROKE EXHAUST VALVES Piston manufacturers use the stao dard of stamping an arrow on the crown chat points coward the front of the bike or the exhaust port Another indication of the correct piston position is co have the ring centering pins facing the intake side of the cylinder There is only one motorcycle known that lve ever opposes this rule the carly model Kawasaki KDX 175 That model has a piston with ring centering pins aligned on both sides of the exhaust pore 18 Four Stroke Contact This piston is from a modern big bore thumper The cam chain tensioner failed allowing the cam chain to jump a few teeth on the camshaft which allowed the exhaust valves to contace the piston This can also be caused by any ott problem thar allows for valve fl ok slack cam chain For example the Qhain guides can wear out the ch ar and the cain or crank sprocket automatic cam chain rs can fail abruptly Valve float cur when th
25. you need to CYLINDER PI hoe opening This also retards the bring the bike to peak stock condition The cylinftee ports are designed to exhaust port timing shortens the before you add aftermarket equipment protlak a ertain power characteristic exhaust port duration and increases This section lists each performance gvef y narrow rpm band Porting or the compression rario mod and describes how to modify each San is a metal machining process 2 Narrow che transfer ports and reangle system for the performance you wane Qe p formed to the cylinder poris exhaust chem with epoxy to reduce the pore These are silicon molds of popular combustion chamber Gesigns starting from the left flat top domed and hemispherical designs Flat top chambers are good for motocross and off road because of their quick throttle response and use of lightweight flat top pistons Domed chambers are better for thermally loaded high cpm engines Most factory 125s use domed pistons Hemi chambers are used for road racing and shifter karl applications 135 TWO STROKE ENGINES The latest performance trend in cylinder head design is the two piece head The water jacket housing and combustion chamber are separate pieces bolted together on the cylinder Bronze combustion chamber 2re used because the material transfers heat efficiently and is more resistant to detonation damage Gitar designs and compression ratios can be quickly changed for different applications
26. 43 TWO STROKE ENGINES Cylinder heads with offset spark plug holes must be mounted to a face plate for machining in a lathe Three of the boll holes are tapped to a 3 8 NC thread and spaced from the plate with wrist pins and washers Si 144 This crankcase is being bored on a milling ET Because the crankshaft was stroked for a 2 mm offset the perciease mast be bored 4 mm larger for the proper clearance area where the connecting rod swings ig is O milling casks Ies ma amp yASnithy and is called Granite 1324 Ths machine sells for 3 700 wid age of accessories including hA ool bits end mills collets Ji ter boring and ida i King is considered a finish machining rocess The rwo types of hones used in facing head the motorcycle business are a horizontal machine with a stationary mandrel and a drill operaced portable honing mandrel Common applications for boning include the cylinder bore connecting tod a cylinder head s cam and tapper bore surfaces crankshaft big end bores and valve guides The biggest supplier in the world for motorcycle honing machines and supplies is Sunnen headquartered in Sc Louis Missouri Grinding and Polishing The two types of grinding tools are bench and cable driven Both a G in bench grinder and a hand operated setup like a Dremel can accept a variety of media ranging from rough stones co finc rubberized ahrasives Tn my porti
27. 7 95 RM250 The wedge shaped valve was posirianed ar about a 45 degree angle over the exhaust porr The ATEV system is designed ro regulate che effective stroke exhaust gas velocity through the exhaust port and on 1995 and Jater models ir controls the exhaust gas temperature The ATEV system is self cleaning in thac carbon is scraped off the valves every time they move Some of the early RM models suffered from broken exhaust valves when the stem would detach from inh cylindrical wedge Thar problem cured in 1991 when the radius be the stem and valve was incr two common problems rhat assembling the syseem 1 Too much prelo Khe spring On the let side ay der there isa dial chayty Mepis the spring preload for asr valve system The preload dotsn have a grear effect on the engine s powerband but too much preload will preven the valves from opening which causes lack of top end powcr Crit erossed sping A cenic rting sping ow Wid tighe Side OF mhe cylinder locared on the tod acrnates the valves This spting is commonly iosralled incorrectly The spring tabs N This is a typical layout of the val 2003 RM125 1989 to 1995 RM2 following models 1989 to 2005 RMBO and 85 1988 to RMX madels The cylindrical valves are comprised of a Stationary guide and a sliding knob mounted on the top left side of the cylinder provides spring tension It s common for thaJeft spring to get twisted in half from turning the
28. 88 1992 KX250 and 1990 2000 KX500 are aluminum but have a hard anodized coating that resists wear However the drum valves eventually wear at the drive channels for rhe center wedge valve and the sloppy fit becween the wedge and drum valves prevents the cenrer valve from fully opening That is why the bikes ger noticeably slower as they ger older There is no preventive cure or aftermarker fix You just need to replace the drum valves when the drive channels wear out The 1993 KX250 was the first year for the KIPS system used through present day models The system uses a single wedge and flapper valve for the main exhausr port and two drum shaped valves for the sub exhaust ports The valves are linked together with two rack and pinion units on the right drum valve and a steel gear on the upper rack linking the wedge valve A left hand thread nur retains che gear ro the rod chat actuates the wedge valve Check the EXHAUST VALVE TIPS AND TUNING This 1s the new generation KIPS system used on the 199 models There are three pieces a stationary guide a self cleaning and should be checked every time a sludge to accumulate under the valve cover X nut periodically if the mysafOgsens the wedge valves become able The KX250 KIPS also es two large isMipacion of the cavities to allow f iopemite Mar travels back up e at low to mid rpm Ies nt that che rwo valve covers on import the cylinder are s
29. LOGBOOK Keep a logbook chat tracks the number of riding days and che periodic mainre nance From reviewing che log you will Icarn how often you need to service the cop end if you record the measurements of the cing gap and rhe piston diameter A logbook also gives you greater leverage when you cry to sell your used bike for a premium price BIG BORE KITS One of che best ways to increase horse power is to increase displacement by overboring the cylinder This can be ideal for play or vet class riders where the increased displacement won t be illegal for your race class When done right a big bore kit can give you more power cverywhere rather than an increase in only the cop e bottom of the powerband S cae are typically more GO give you more power whel need it Piston ryanutacturers such as Wiseco njoren piston kits for popuj ted bikes These kits boost the Keren of the cylinder to the limig ofa racing class or ro a larger splacemenr class for example 80 cc t0100 cc 125 ce co 145 cc 250 ce ro 65 ce or 300 ce and 495 ce to 550 cc Riders compering in the AMA veteran class can ride a bike with any displacement Riders competing in hare scrambles and enduro can race rhe 200 ce class with a 125 converted to any displacement AMA motocross and enduro racers can make the 250 cc bikes legal for open class by increasing the displacement a minimum of 15 percent to 286 cc Also you should ar lease consult with an e
30. NDER All cylinder bases use aligning dowel pins around two of the cylinder base studs The pins are made of steel and after heavy power washing they get corroded That makes ir difficult to remove the cylinder from the crankcases Never use a pry bar I har will damage the cylinder Insread use a plastic mallee to hit upward on the sides of the cylinder ar a 45 degree There are two ways lo install the top end You can install the piston assembly into the cylinder and then lower the piston and cylinder anto the connecting rad to pin it The conventional way is t0 install the piston assembly on the rod and lower the cylinder onto the piston Never twist the cylinder ar rings when siding the cylinder and piston together The ring ends can get trapped in the ports and be prone to cracking angle Alternate from left co right sides to lift the cylinder up evenly After you remove the cylinder stuff a shop towel into the open crankcases ro prevent debris from entering the engine THE DIFFERENT TYPES OF STEEL LINED AND PLATED CYLINDERS There are two types of cylinder bores used on dire bikes steel or cast iron sleeves or those with plaing on the aluminum Most dirt bikes made after 1989 have placed cylinders You can check the cylinder ype with a magnet If the magner sticks to the bore it is a sleeve Fit doesn t stick it is plared 101 TWO STROKE TOP END REBUILDING This s a view looking down the exhaus
31. TUNING GAUGES engine we are really talking about a mixture of fuel and oil If you richen the pre mix ratio 20 1 as opposed to 30 1 there is more oil and less fuc in the same volume of liquid which effec tively leans the air fuel ratio This fact gives the clever tuner one more tool to use when the correct jer is noc available or when none of che standard jets are exactly tight You can richen the jetting by slightly reducing the pre mix racio less oil You can lean the jeuing by increasing the pre mix ratio more oil Changes in the pre mix ratio affect the jeuing over che entire throcrle opening tange buc the changes in ratio must be small co prevent excess wear from lack of lubricating oil or fouled plugs from roo much oil Pre mix oils are formulated for a fairly narrow range of pre mix rarios You should examine the oil borde for the oil manufacturer s suggestion on the pte mix ratio All production two stroke dirt bikes have a sticker on the rear fender suggesting that you set the pre mix ratio to 20 1 Thac sticker is pur there for legal purposes Always refer to the oil manufacturers suggestion on pre mix In general small displacement engines require a richer ratios premix ratio chan do large displace ment engines because smaller engines have a higher peak rpm chan larger engines The higher the engine revs O more lubricarion ir requires A TUNING GAUGES There are three cypes ofapdug
32. The head s squish band recess gap has been increase so the piston doesn t contact the head form the three surfaces However the crank cannot be rebuile by tradicional means because of the difficulcy with indexing the crank halves for the proper stroke dimension BALANCE FACTORS THE MAGIC BULLET Truing the crank refers to the process o o include adding weighPadaYlenser mass to the bortom A rh trank halves or lightening the b ves ar the cop Substitute pagtecials for adding weight lead and osmium ROD KITS AND SPACER PLATES were popularized by the motor nee Truing and balancing are often confused AG rms long rod kit and spacer aligning the crank halves abour the Qiy end pin making the halves paral each other Balancing refers co the balance facror which is the reciprocating mass to o mass The reciproc Nags consists of most of the KO rod and the piston assembl rotational mass consists of thi Ives Crankshalis are light The top near the big end pin usina number of different methods Sometime there are holes drilled in the crank halves or lighter materials are substituted Crankshaft balance is important because excess vibration is converted co friction and heat which spread throughouc the crankcases and reduce the charge density And chat directly affects the peak power Altering the balance factor may 138 ycle press Here are some frequently as
33. a PDA and change things such as the engine s ability co perform on less expensive fuel The Kawasaki Keihin thtotele electronic carburetor with position sensor is ideal for computer control and coordination with things such as exhaust valve systems There are less expensive aftermarket parts such as igniter boxes FME sells the Wolf brand igniter box chat featurg adjust on the By timing Products gad as these offer a choice of an gae curve for bercer midrange y ne wiyh a retarded curve for bepeNgver rev ereere r pap the saine price as gniter box PERFOR ACHINING TOOLS Al CHNIQUES MACHINE TOO There are four different machine tools used for perforinance machining of motorcycle parts lathe vertical mill horizontal or portable hone and grinding or polisbing tools Lathing A bench lathe can be a handy machine in a motorcycle workshop A 9x20 in bench lathe costs as lic 95 new Mes such as This most of the from mail order c p ENCO Hast simple lathe paard and reighr tasks neede haintain dirt bikes or a lathe include resur stroke heads Commonal facing cylinder nders and carbs machining fing grooves turning cylinder beri KA Kia polishing axles and checking the ueness ofa crankshaft Tooling for lathing indudes the cutting tools and the mandrels or fixtures needed to moune 2 wotk piece For centered two stroke heads a threaded mandrel can be
34. alve operation after you assemble che rop end by using the inspection cap on the left side of the cylinder The CRI25 HPP system was redesigned in 1990 Honda chose to use a system similar to the 1986 co 1991 CR250 featuring horizontally sliding valves This system has been plagued wich problems over the years I he valves are prone to carbon seizure hecause che critical square edges face the exhaust stream If the clips that fit on che ends of the valves vihrate off ot if the valve wears too much then the valve can tlt on an angle and serike phe Another common rel K ben happens when tuner i exhaust ect to grind port ducing porti sass ePorners for piston clearance fee again the valves strike the oka they protrude js ra Honda made a modifica P tothe valves they added an aped rib thar prevents the valves piston m angling in and contacting the iston The problem of clearance berween the top of the valve and the guide was eliminated and the new style valves provide more low end power The valve and guide sets from the 1998 to 1999 models fic che CR125 models back to 1990 In 2000 Honda redesigned the CR125 engine and adapted the exhaust valve system uscd on the RS250 road racer Honda also used this sysrem on several dual sport and street hikes sold in Asia and Europe The new system is simple and effective It is a wedge shaped valve that pivots at one end similar co the CR250 The valve is much rh
35. am spindle and the system won t work at all so don t be a hammer head The best way co check che HPP system is 10 remove the left side valve covet from the cylinder start the engine and warm ir up and then rev the engine The valves should be fully closed ac idle and fully open when the engine is revved In 1992 Honda introduced the HPP system currently used on the CR250 The system features a center valve for the main exhaust port and two oe drum valves to control the flow ol aO sub exhaust ports The syste features a return of the old t used on the mid 1980s Lhe resonator improves the response and mellows che and at low rpm A thin nR che valves together 4 Whole system is mostly self B ping to prevent carbon buildup The inside of the center valve has an elongated passage where che tie rod travels This elongated passage is prone to carbon buildup over time one to two years The carbon limits the range of movement in the valves The carbon is easily removed hy using a small diameter rat rail file I he sides of the cenrec valve and the drum valves interface and that area is prone to carbon buildup as well A wire brush or file is an effective tool in cleaning the exhaust valves Here is a simple way to check the operation of this system On the left side of the cylinder there is a 17 min cap bolt thar exposes a straight line matk in the left drum valve There is a corresponding mark on che cyli
36. arburetor works beret for high rpm powerbands For 125 cc engines a 34 mm carburetor works well for supercross and enduro and a 36 co 38 mm carbu retor works best for fast motocross tracks For 250 ce engines a 36 mm carbureror works best for low co mid rpm pow bands and a 39 5 mm carburetor best for high rpm powerbands a Recently there has been y in the use of air foils sn ih diog for carburetors These Cases ate throttle openings ania as Perform Mrcering in Florida offer a we overbore the carb and include insects Yo reduce che diamerer of the cabureros Hor example a 38 mm carb for a 250 cc bike will be bored to 39 5 mm and two insers will he supplied The carb can then be restricted to a diameter of 36 or 38 mm Not every carb can be overbored The maximum diameter of the carbu recor cannot excced the width of the slide or air will bypass the venruri and the engine will run roo le e are two ways to overbore d by serip ping it down and nitqing W on a lathe Keihin PWK and i cound slide or by milling ic ercical mill usinga rorary table lOs table method is necessa arburetors where the needle not be removed Keihin ni TMX Whenever a carb Se Pred the jetting musr be rich to compensate for che loss of KA velocity AFTERMARKET REED VALVES Like large bore carburetors bi valves with large flow area work best for high rpm powerbands In general reed valves wich six or more petals are used f
37. ase The hand pump produces vacuum pressure up to a stan dard seeing of 5 psi The normal bleed down pressure loss is 1 psi per minute Cylinders with complicaced exhaust valve systems can be difficult co block off air leaks and harder co test Crankease pressure testing kits are available froin Motion Pro IFI suspecr that an engine has an air teak in the crankcases do a visual test Starr by power washing the engine clean Theo remove rhe magneto cover Spray the magnero clean with an aerosol can of brake cleaner Make sure to use a non chlorinaced cleaner Sprinkle baby powder on all che suspect areas of the engine Sprinkle the powder on the crankcase around the magneto ac the crankcase seam line the cylinder base and the reed valve Run the engine for a while the white baby powder will highlight any fluid or aic leaks on che engine The baby powder test is nuch better chan the alternative test of blowing raw propane gas ar different areas of a running engine and listening for a change in the idle rpm Thar is dangerous because it involves flam mable gas and a hot engine with random electrical shorts MAINTENANCE AND INSPECTION DISPLACEMENT 80 cc TEAR DOWN AFTER 5hours When you do a top end job check the condition of the cylinder head The top end bearing of this bike started lo break apart and the needle bearings smashed between the piston and head causing damage to both The head was refin
38. d the big ed drill new holes farther ft center Holes and the crank Bore the b holes larger and TIG weld ric flanges Precis d an eccentric crankpin Manli iring new stroked crank N je most expensive choice and Se fy there are no afrermarker prod ucts of this type for modern dirt bikes Relocating rhe big end pin holes is the most logical and reliable choice The crank is disassembled the holes are TIG welded with stainless steel filler tod and the holes are rough bored and finish honed with the crank halves jigged together to ensure accuracy Eccentric flanges are manufactured by gun drilling rod stock off center and then turning the rod on a lathe into a flange shape The cranks big end holes are bored oversize and the flanges are installed indexed and TIG welded ro the crank halves Cylinder base shims are used for three reasons to accommodate stroker cranks and long rod kits or to advance the port timing Eccentric big end pins were popular ized in Germany 20 years ago The German logic is to avoid heating welding material co the crank halves so as to mini mize stress and distortion Oversize rod stock is OD outside diameter ground to This spacer plate was custom made by Cometic Gasket Co Average cost is 50 137 TWO STROKE ENGINES This is a wew of the head cylinder and piston on an engine where the crank has been stroked Notice how piston crown extends over the top of the cylinder
39. der heads thar have different cartridges to give different cylinder head shapes The various hcad c g rtridges bave different combustion bowl shapes compression ratios and MSV ratings The head cartridges arc incrementally different corresponding to powerbands ranging from extreme low end to high rpm CRANKSHAFT STROKING Stoking refers to 2 combinarion of metal machining processes thar relo cates the center of the rod s big end in relation to the crankshaft center A crank can be stroked or de stroked Generally speaking stroking refers co increasing tbe distance between the big end and crank center and de stroking tefets to reducing the distance Stroking increases the displacement of the engine and de stroking reduces the displacement PLAYING THE NUMBERS Certain combination inder bore size crankshaft Nand connecting rod length pi a powerbands for certain apppeations In 125 cc accepted standard is a macocros 3 bore olaS4uxim a stroke of 54 5 mm apd aning tod length of 105 Maf Ya road race it s 56x50 and 110 A hort stroke enables a higher rpm before critical piston speed is attained 4 500 fect per minute thars why a shortet stroke is used for toad racers These engine configurations are cermed over square because the bore is greater This is Wiseco GP piston designed for ported cylinders it is a high performance piston for a 125 cc engine Unlike a stock
40. different hop temperature correct provides a greaty than just swappj up company he temperature inside he is affected by che carb jetting R I formulacion and ignition timing curve Cylinders with exhaust valve systems that employ large cavities or venting can also dramatically affect how the pipe works Some of these things you can control others you can t How Aftermarket Companies Design Pipes You re fooling yourself if you think thar the pipe manufacturers run comprehen sive R amp D programs on every mode of 140 This is a comparison between two pipes for a 129 band It has a short tuned length and sane ine The pipe on top is designed for a high rpm power gles The other pipe is designed for a low to mid rpm_ powerband A long tapered head pre ay sh llow cone angles highlight the difference from the rev pipe O ea pipe for ich bringing a dirt bike that th They are challe large variety ol to marker quickly and the w they do it is simple ioia siyen onthe Then rhey decide which Ahe powerband they want co They sta e Nexe they cut the front and D KS cones off the pipe and weld spring joops Then they interchange a selection of angled cones and make dyno runs Some companies use the same cones on all their 125s or 250s since the Japanese manufacturers use ncatly identical bore stroke rod length and port timing Once they are satisfied with the cone choices they
41. e engine is over re break Mosprt aggressive amp wy Wear valve springs this happens when an is installed without 19 Reading Piston Burn Patterns Reading che burn patterns that naturally form on the crowns of pistons cao give a tuner insight into several aspects of the engine s performance and condition I he color and arrangement of the bum patterns and the location with tegards co the different types of ports provide insight into what types of changes will make the engine run its best The piston shown here is from an RM125 The bike was running a bit weak throughout the rpm range These are some of the observations of che burn patterns and how they relate to the engine s condition and carb jerting 1 Outer edge The light gray color indicates lean carb jetting Mocha of the left side transfer ports need co be adjusted co match the right side TWO STROKE EXHAUST VALVES Three words sum up exhaust valve maintenance spoogey goocy and grungy If two stroke exhaust valves didn t have such a dramatic effect on the engioe s powerband l m p f mechanics would remove them eat them to bits with a hgffimbcAn frustration There is little ihfoArion given by the morem on how to diagnose and ia repair the ft valve systems on well edlir i us its This sacdow provides a gil KG ghirnceeiistic miethnateal prob Jenys AXt occur in exhaust valve systems brown js the optim
42. e flyweights aren t parallel and must be rotated about the crankpin This is accomplished by striking the flyweight at the exact point of the greatest runouc with a brass hammer while holding che opposite flyweight in hand Te doesn t take much force to rotate the flyweight into true Now you can understand why we must be careful how che crank is installed into che crankcases during engine assembly Ie is very easy to throw the crank ouc of true if you bear the hell out of it and the cases during assembly This is the RCE crankshaft assembly jig This excel lent tool features a sturdy jig frame and bushings to align the crank halves and press the crankshaft 131 Changing the powerband of your dire bike engine is simple when you know the basics A myriad of aftermarket accessories is available to rune your bike to berer suit your needs The most common mistake is ro choose the wrong combination of engine components making the engine run worse than a stock engine Use this section as a guide to inform yourself on how changes in engine components can alter the powerband of your bike s engine Use Tuning Guide to Performance the Modifications section ro map out a strategy for changing engine compo nents to create the perfect powerband TWO STROKE PRINCIPLES Although a two stroke engine has fewer moving parts chan a four stroke engine a two stroke is a complex engine with different phases taking place in che cran
43. e of the bike and lose thermal efficiency and horsepower Placed cylinders are harder and last longei chan sleeved cylinders Kawasaki cylinders with the original Electrofusion coating or hard chromed cylinders can be repaired with nickel plating or sleeving Steel or cast iron sleeves cannot be nickel plated unless they are separated from the aluminum cylinder The pretreatment or the plating would disintegrate the aluminum There are four companies that re place cylinders in the United States The average cost to te plate a cylinder is abouc 200 THE PISTON Some uofortunate guys do more damage replacing the piston than che actual wear on the piston Remove the citclips with small needle nose pliers and throw chem s It is a common mistike to reuse lips but the cheap spring steel wire clips will fatigue and break if you install chem for a second time away cil After removing the circlips you have to remove the piston pin Never use a hammer and punch to remove the pin Thar will damage the connecting rod and needle bearings Instead use one of the pin extractor cools available from your local Franchised motorcycle shop You can also grasp the piston with one hand and use a 3 8 in socket extension to push the pin oue with your other hand Too many people replace their pistons too often The exact service interval for your bike depends on how hard the bike was run for how many hours che quality of the lubrication
44. e particles to bond to the cylinder wall The particles are very hard and sharp they don t bond to the ports so the manufacturer ot recondi tioning specialist muse thoroughly clean the cylinder Sometimes the silicon carbide flashing breaks loose from the ports and wedges between the cylinder and the piston This causes tiny vertical scratches in the piston This problem isn t necessarily dangerous and doesn t cause catastrophic piston failure but it should be addressed by thoroughly flushing che cylinder and ball honing the bore to rede fine the crossharching marks This type of problem can also be caused by a leak in the air intake system Debris such as saod ot dirt can cause the same type of tiny vertical scratches in the piston skire and cylinder wall However the main difference is the color of the piston crown and spark plug Dirt will leave a datk stain on the piston and sand will make it look shiny like glass That s because melted sand is essentially glass Normally you will need to replace the piston kit because the scratches will reduce the piston s diam exer beyond the wear spec 4 N 12 Burnt Out Blow Hole This piston overheated so badly thar a hole melted through the crown and collapsed the ring grooves on the exhaust side Normally the piston temperature is higher on the exhaust side so catastrophic problems will appear there first There are several reasons for a failure like this Here are the
45. e steel alignment pins with steel wool and penetrating oil Fxamine the pins makes the p accen out at high closely If they ate deformed they tpm Theaglajvely long spark lead won t allow the engine parts to bole time enai greater pressure rise in cogether tightly which can cause a r before che piston reaches is produces more torque in GeyMidrange bur che high pressure to pumping losses ar tremely high rpm dangerous air leak or a coolant leak The pins are about 2 each Replace them if they re rusty or deformed 3 Never reuse old gaskets Remove them with a razor blade or gasket scraper Dont use a drill driven steel wool pad to remove old gaskets because the woo pad can remove aluminum from the cylinder and head which will cause a gasket to leak 4 Always check the cing end gap on a new ring by placing it in che cylinder between che head gasket surface and the exhaust port The gap should he 0 012 co 0 024 in 5 Always install the circlips with rhe opening facing straight up ot down chat way inertia will hold it cighe PIPE AND SILENCER Because only the bore size is changed you won t need a longer pipe only one with a larger center section FME s Farcy pipes work great on engines chac have been overbored HEAD GASKET Increase the bore diamerer of the head gasket to the dimension of the new piston If the head gasker overlaps into the cylinder bore more chan mm on each side it could conract the piston o
46. e that prevents the valves from wearing and contacting the less expensive than previous models and fit back to model year 1990 horizontally chrough a valve guide The system was plagued with a mixture of design problems and misinformation on how co service and complicated exhaust arcange mene This section lists some common re time the valve problems and some tips for timing the system installing the cylinder and engaging the HPP mechanism Common HPP Problems wo main problems plague the HPP system carbon fouling and rack and cam spindle damage Ihe square shape of the valves contributes to the accumu lacion of carbon in che corner of the valve guide stationary pare that is directly in che exhaust gas stream and this causes the valve ro become carbon seized Chamfeting che corresponding edge 1 mm of che valve will eliminate this problem The cack and cam spindles are easily damaged when the cylinder is installed incortectly or the HPP mecha nism is engaged incorrectly See the photos for examples of damaged rack and cam spindle pares Thus is the CR125 2000 exhaust valve system it s similar to the design proven on Honda s line of road racers and two stroke sport bikes n Japan Two main valves are actuated from a single pivot to accurately vary the exhaust port s width and effective stroke This system has a chronic problem of carbon seizing so you can t remove the shalt from the valves to service them
47. ealed with gaskets and it is normal for large amounts of black sludge to accumulate under those valve covers Jt rakes years for che sludge to accumulate ro rbe point of adversely affecting performance T he only way ro clean our the sludge is to have the cylinder hot rank cleaned at an automo tive rebuilding store KX80 100 and 1998 to 2002 125 oS and a pivoting flapper This design is mostly the top end K s normal for a large amount of The 1993 2000 KX250 wedge valve tends to form burts ar the ourer edges that face the piston The burrs prevent the wedge valve froin opening fully and the thin flap that comprises the exhaust port roof hangs our into the exhaust gas stream producing a shock wave that closes off the exhaust part File the burrs until smooth and check the wedge valve through the full range of movement The valve pocket in the cylinder gets worn roo ARtermarker cylinder rebuilders such as Max Power Cylinders apply a hard coating to that area to reduce wear and build up areas that have wom down fom the moving wedge valve The 1998 to ae 85 and 100 use this chum wih to actuate the KIPS valves The easies service the top end is to start by remaying the bolt that retains the pinion shaft and ift the shaft off with the cylinder To install the Gros Close the valves and align the dot on the shaft with the open bolt hole or the valves won t work because they won t be timed correctly Another c
48. ed valve Bikes with single reac shocks have offser carbs The RAD valve is designed to evenly tediscribure the gas flow from the offset carb to the crankcases A RAD valve will give an overall improve ment to the powerband Polini of Iealy makes a reed valve called the Supervalve Ic fearures several mini sets of reeds positioned vertically instead of horizon tally as on conventional reed valves These valves are excellenr for enduro riding because they improve throrde response l sts on an inertia chassis dyno show the Supervalve to be supe rior when power shifting However che valves do nor generate greater peak power chan conventional reed valves THE SECRET LIFE OF A TWO STROKE EXHAUST PIPE A pipe job is to draw out the burnt gases from combustion along with some unbumed gases and then reflect a wave thar plugs che unburned gases back into the cylinder before the piston closes off the exhaust port The pipe is comprised of five distinct sections header diffuser center dwell convergence cone and stinger There are algebraic formulas for 139 TWO STROKE ENGINES FMF makes tailpipe silencers in three basic varia tions short for low end long for top end and a spark arrestor for sound and fire safety the dimensions of each section othe pipe but they are only a baseli designers The problem is rhay exhadst gas and outside skin tem re are variables to consider ing the A the pipe g advantage a
49. edriven tools to tool bits 1a polishing supplies They even sell uring tools and flow benches A VALVE SEAT maune roos S The latesr trend formance machining is che ngle carbide cuter that floa Q valve pocket holding a grie path Serdi and Newen selNomplere machines from bench top micro to heavy duty production machine tools Goodson Automotive a These are tool handles and textures Tool hy fer supply company markets an usive line of multi angle cutters from famous tuners such as Tony Mondello On four stroke heads much of the porting is accomplished by getting the right carbide cutter profile for the application and valve size Goodson also sells a floating rool holder designed for the R 8 collets used in most vertical milling machines The tool holder sells for 900 and the carbide cutters for 40 each Vhere is hardly a need to resurface a valve because mosr of the hard coatings are applied during manufacruring D right angle designs These tools hold tool bits of different shapes materials lug into cable drives CC Specialty in Lawrenceburg Tennessee 1s the biggest supply company I bits and polishing supplies This is a Serdi Micro multi angle valve seat refacing tool Multi angle seats offer improved flow and longer valvetrain life 145
50. emperatures of machine cools change through production runs and machine operators have inconsistent performance The Japanese manufacturers have two to four different sized pistons and cylinders normally labeled A B C and D If they only had one size the piston to cylinder wall clearance would vary berween 0 001 printed or stami mass produge and 0 006 in In the standard Japanese alpha labeling system A denores che smallest bore or piston size and every lerver after thar is slightly larges usually in increments of 0 0015 in If you try to put a D piston in an A cylinder the piston to cylioder wall clear ance will be so tight char a seizure might occur Measure the pistan at the bottom from front to back Pistons are cam ground and tapered specially for engine running condi tions A 30 caliper is good enough to get an accurate measurement of the piston 103 Pro X piston kits come in increments of 0 0005 inch for most modem Japanese dirt bikes LA Sleeve distributes Pro X and more ndiudual top end engine components than any company in the word PRO X OVERSIZE PISTON KITS Pro X is a marketing company thar sells surplus pistons from the Japanese company ART which makes all the cast pistons for the Jap manufacturers These pistons are t same quality as che OEM pistons they are available in sizes larger tha alpha pistons available from frarhised dealers The Pro X ie Oe ally se motorcycle
51. engine is 12 utilizes a lambda seag dN is inserted haust stream from the piston in a four stroke and in inro the center of approximatel the header The number one cause of stuck throttle condition is a bad throttle cable Improper routing caused this cable to fray above the carburetor The cable should route through the center of the head stay brackets the baffle cone of a two stroke engine A permanent female pipe fiting 1 4 in musr be welded to the side of the exhaust pipe in order to fasten the sensor I he weld on fitting setup is also used on the temperaiure gauges and the fitcing can be plugged with a 1 4 in male pipe fitting when che gauge is nor in use This gauge is idegl for four stroke engines EGT Gauge measures the ten of the gases in the exhaust pipe hy ans of a tempera ture probe oe into che exhaust pipe 6 inpfighwthe piston This type of gaug oe you to tune the carb jerri the pipe togecher taking adyneage of the face thar exhaust pipes re designed wich a precise temperature i mind An exhaust pipe is designed co return a compression wave to the combustion chamber just before the exhaust pori BGT gauge closes Most pipes are designed for a peak temperature of 1 200 degrees Fahrenheit Mose dirt bikes are jetted too rich which prevents the exhaust gases from reaching their design temper ature so power output suffers Sometimes just leaning t
52. es that professional tuners use En jerting 1 Retative air densi D gauge 2 Air fuel AF ga jo Prete 3 Fxhaus perature EGT gauge The folldwgng is a description of how each gauge funttions and its advantages RAD Gauge A RAD gauge is the best choice for dirt bikes because of the convenience bur che gauge is no good unless you get the jetting perfec The RAD gauge provides you with an indi cation of how much the changes helping you compensare for the effects of changes in che air emper r density aure altirude and baromerric pressure The gauge is calibrated in percentage points Once you set the jetting with the ride and feel method you can set the calibration screw on the gauge so the needle is pointing to 100 percent When the air densiry changes the RAD gauge will show the relative percent of change Using a calculator you can multiply the percentage change shown on the RAD gauge by tbe jer size and determine the corrected jet size for the air density The pilot slow and main jet have number sizes that correlate with the RAD gauge buc the needle clip position can only be estimated Normally for every two main jet incre ments the needle lip mnsr be adjusred one notch AE Ratio Meter Lhe AF merer measures the percenrage of oxygen i the exhaust gases and display amp approximate air fuel ratio of aaa The gauge displays AF ratigs to 16 1 The opimum for a F gauge ewo stroke
53. fearures a power valve chac marks a significant design change from the company that pioneered the use of exhaust valves on two stroke engines Looking more like a Rube Goldberg device che new power valve has separare valves for the main center and sub exhaust ports sides the whole assembly is controlled by one actuating rod but the side valves open after the main exhaust valve The side valves are controlled hy two wedge shaped ramps that resemble the shift drum from a rransmission The tamp design offers versatility in runing By changing the shape of che ramp the duration and timing of the sub exhause ports can be changed to match a rider s ability or the demands of the terrain So far there are no aftermarker compa nies making these ramps bur you can 121 TWO STROKE TOP END REBUILDING ber chat the factory teams are expeti menting with them The stop plate of the center valve tends to crack allowing che valve to contact the piston Look for cracks in the plate right around the two retaining bolts KTM 250 300 360 380 1990 2004 KTM uses two distinct designs of exhaust valve systems Re uses one large cent ia with the actu ating tod casy Two drum valves control the lt S ports and steel gears pifia the main and minor al x effective stroke can be BY altering a stop plate on che of the cylinder The governor gonttol in the right side case has an TA inspection cap that allows tuners to add thin washers and
54. fter you ride the bike reduce the risk of dirt and debris falling into the engine Once you remove the cylinder stuff a clean rag down into the crankcases 2 The cylinder and head use alignment pins to hold them straight in posi tion from the crankcases on up The pins make ic difficult co remove the cylinder froin the cages and che head ftom the cylindg Spmetimes the steel aligomen orrode into the aluminu eggs components Try spraying eogMating oil down the g studs before atcempting end overrev Overrev is a slang term to the cylinder and head thar describes the uscable length of the use a flat blade screwdriver powerband at high cpm Ge or meral hammer to remove The scientific reason far che shift of y the cylinder Instead use this rech the powerband ro extremely high tp nique buy a lead shot plastic mallet is because che temperature in the increases with che retarded tinting apd swing it at a 45 degree angle upward against the sides of che cylinder that enables the pipes ened to be more synchronous SO piston speed and pore iming vlinder Alcernate from left to righe hiring the sides of the cylinder co separate it Advancing the Tncreases the midrange hic ofSghe Powerband but IGNITION TIMING The ignition timing has a minimal effect on che powerband Retarding the timing reduces the hit of the powerband in che midrange and extends the top from che cases evenly Clean th
55. haracteristic problem of che KX250 KIPS is broken governor levers The lever char rransmirs the movement from the centrifugal governor to the right half The piece is located under the right side cover IF your KX250 suddenly loses le case lever tends to break in top end power its probably due ro a broken actuating lever or the carbon seizure of the KIPS valves 1988 1992 KX250 and 1990 KXS00 KIPS The explanation of this procedure as written in che Kawasaki service manual 2000 Timing Procedure is confusing Ic requires you to time the upper and lower racks at the same instant My method of timing the exhaust valves uses simple steps that enable you to check your work as you go The 1988 1992 KX250 and KX500 use the drive channel system to acruare the center valve Here is the bese way to time the KIPS on these models L Sec the cylinder upside down on a bench Install che center valve bur don t bolt icin 3 Install the side drum valves and align the drive channels on the drum 117 TWO STROKE TOP END REBUILDING valves with the ceater valve but don t bolt it in Lift up che drum valves so the bouoms of che gears are flush wich the cylinder base lake care noc to disengage the center valve 5 Slide in the rack from either side of the cylinder Posirion the rack by installing che seal pack and pulling the rack out until ir bottoms against the seal pack This is rhe full open
56. he device has a plunger that installs circlips without the stress caused by pliers sembling them Caution Oven cleaner arracks aluminum don t leave it on the cylinder for more than 20 minutes Oven cleaner can be used on both steel and placed bores The oven cleaner will help loosen the oil glazing on the cylinder walls hen you can use a Scotch pad to hone J jn a criss Wear rubber n you Nand flush the cylinder ard with soapy water to p a al aner and break the molec use oven ular bond of the oil 90 the Libris can be rinsed away Sleeved cylin ders especially Kawasaki cylinder bores with Electrofusion coating are vulner able tn corr n after cleaning Spray some penertating oil on che cylinder bore to prevent it from rusting Caution Certain types of cylinders corrode quickly after the cleaning process so spray the bore area with penetrating oil to displace the water HONING THE CYLINDER BORE Many people c mail me with questions regarding haning cylinder bores If you want to buy a hone to deglaze bores or polish off small scratches a ball hone is the best choice Ball hones are manufac cured by Brush Research in Los Angeles under the brand name Flex Hone These hones are available under differenc labels and are most casily avail able fram auto parts stores Buy a sive that fits in che tange of the actual bore size Hones are available in several different materials and gri
57. he main jet and the neeclle dip position makes a dramatice difference Digitron is the most popular brand of FGT gauge le measures both FGT and rpm The gauge is designed for kare racing and is nor suited for wet weacher Te is designed 10 mount on the handlebars so the rider cro focus in on it Once you perform the baseline jetting send the rider out on the bike with the FGT The rider observes the EG to give you feedback on the nece sary jetting changes Once th dialed use the tachometer to check the condirio jetting is peak rpm of the engine on the longest straight of the racetrack For example if the peak rpm exceeds the poine of the engines power peak rpm change the rear sprocket to a higher final drive rario rear sprocket wich fewer teeth until the rpm drops into the target range An EGT gauge is ideal for dire track bikes and karts where peak rpm temperatur is critical 99 Top end rebuilding is che mose frequent and costly service routine on two stroke dire bikes Every year dirt bike riders waste loads of moncy on top end pacts that didn t need to be replaced or they make costly mistakes while performing repairs This section provides the dos and don ts to easy top end rebuilding plus some tips that arent printed in your factory service manual BEFORE YOU START Thoroughly wash your bike because dirt stuck co the underside of the cop frame tube could break loose when servicing and fall
58. her In che Beld of unsteady gas dynamics rhat is called super position A multi phase resonator has a specific volume O and orifice size that allow the reson to have an effect based on rpm andir position in the expansion chan amp t bike maga Whats a Titty Pipe Tm waiting to flip ope zine someday to Pipe T chin age teenager who reads chese ould embrace the concept Rand Nouis an engincer at GM Racing invented the multi phase resonator nicknamed the city pipe It looks like a bulging chamber chat fastens to che pipe Some modetn Canadian snowmobiles and Iralian trials motorcy cles use them in production Basically the multi phase resonator consists of an oval flask with an orifice that intersects with the edge of the pipe It serves to add volume and length to a particular section of the pipe The orifice size determines the rpm at which the resonator will func tion Honda used a similar device on R125 and 250s on the 1985 and 1986 models The concept was abandoned because che burerfly valve used to control the orifice of the chamber frequently clogged with carbon How the Ignition Changes the Pipe Though ic doesn t actually have anything to do with che pipe an adjustable ignition can have a great effect The ignition boxes have two timing circuits built in A switch is provided to go from one circuit to the other One timing curve has a steep advance for good low end power and quick throttle
59. icker and can vary the exhaust ports effective stroke time area and duration ovet a wider rpm range Its a_self scraping setup so maintenance should be greatly reduced ovet previous models KAWASAKI KIPS Kawasaki s KIPS exhaust valve system has gone through steady design ccfine ment Kawasaki uses a different system co suic the needs of the different model bikes The earliest KIPS design used two drum shaped valves co control the flow of the sub exhause ports Opening the ports gave rhe exhaust pore more time atea The main exhaust port was celatively small with modest timing and duration A rack and pinion setup 115 TWO STROKE TOP END REBUILDING This is common problem for all KX and KDX models that use aluminum drum valves the gear teeth tend to shear when the valves become carbon seved AIL KX250s use a yoke lever like this to tre linear motion of the centrlugal gover actuator rod If your bike starts rui il unbolt the plastic cover on U ight side of the cylinder and make sure t luator rod is moving in vio If not the yoke lever is probably a controlled the drum valves opening tbem ar about 6 000 rpm Kawa used the rack and pinion design in all of its KIPS systems except the 1998 and later KX80 cc and 125 cc models The 1992 KX125 and KDX used the nexr generation KIPS fearured a center wedge valve wich owo side drum valves engaged to a rack and gear actu The which aing sys
60. increase the spring N preload co effect a change in engagemenr earlier model ist IRsi Use RTV Ultra Copper silicone sealant on the front exhaust plate of the KTMs rpm The system is prone to carbon seizute of the srecl valves sub exhaust This is the exhaust valve system for the 1998 to 2005 KTM125 and 200 models Overall the system is very reliable and performs well 122 EXHAUST VALVE TIPS AND TUNING The KTM system is easy to align with dots on the valves aligning with lines on the rack ports Also there are rubber O rings that prevent oil from leaking ouc the sides of the actuating rod thar eventually wear out In arder to service this exhaust valve system you need to remove the cylinder The main valve and ali irs hard ware can remain bolted together here is an access cover on che front of the cylinder and four bolts fasten the cover to the cylinder There is no gasket for the cover it seals with a non drying liquid gasket like RTV silicone The main valve pulls straight out The drum valves are held in plage By pra piatek is Uae ma Phillips screws There is Afc procedure for removing screws Stare by heating the scy s for owo minutes with a proj och to break down the Gan the threads Then use a impact with a No 2 om Phillips tip If you strip the he screws most people do in use a tapered point pu screws our Ir will de the screw buc you should rok hillip
61. inder upon inscallacion 9 On cylinders with ceed valves and large oval intake ports cake care when installing che piston assembly in the cylinder The tings ace likely to squeeze out of the ring grooves Use a flac blade screwdriver co seny push che tings back in the grooves intake port 10 For steel head gaske couod side of tbe Don t use liqui aerosol spray g For hy gaskets steel rings facing down 11 When you initially stare the engine afier a rebuild manipulare the choke to keep the engine rpm relatively low Once the engine is warm enough to take it off choke drive the vehicle around on flat hard ground Keep it under 2 3 throtle for the first 30 minutes Two common myths for proper engine break in are A Set the facing up t sealer use ve sealer instead fiber steel_ ring head ce the wide side of the engine at a fast idle stationary on a stand B Add extra pre mix oil co the fuel When the engine is on a stand it doesnt have any air passing through the radiator and can run too hot When you add extra oil to the fuel you are effectively leaning the cath jetting This can make the engine run hotter and seize TROUBLESHOOTING PISTON FAILURES Examining a used piston can give a mechanic helpful information on the condition of a ewo stroke engine When engine failure occurs the piston is likely to take the brunt of the damage Careful examination of the piston can help a mechanic trace the
62. ished on a lathe MAINTENANCE AND INSPECTION A thorough top end rebuild requires removing the reed valve cylinder head and cylinder You should tear down your cop end periodically and inspect the teed valve cylinder head cylinder piston and so on Use the following chart to determine when you shou tear down your bike Note that air cooled bikes shuld be inspected inore frequently AKigo Ypu may want to inspect more N ou are riding in fine sand or id When you tear down the mo inspecr each system and lox he following aris ed REED VALVE Check the reed petals for open gaps beeween the sealing surfaces In time the reed petals lose their spring tension and the backflow can cause a flac spor in the thrortle response Stock nylon reeds tend co splir ar the edges on bikes that are constantly over revved Expert tiders find that carbon fiber reeds last much longer 125 ce 250 cc 500 cc 10 hours 20 hours 40 hours Most cylinders us jent pins to locate the cylinder on ree If those pins get tweaked Or rusty i difficult to bolt the cylinder to the les Stress fractures will occur on the G Seo the case Check out this hairline crack in the bases below the cylinder bolt CYLINDER HEAD Check the head at the edge of che chamber for crosion marks a sign chat the head gasket is leaking If che head or top edge of the cylinder is eroded it must be turned on a lathe ro be resurfaced CYLI
63. ith changes in rpm and the spool is attached co a linkage system that oper ares the exhaust valves in the cylinder Factory race teams have different combi nations of springs ramps and balls to tune the exhaust valve operation and enhance the powerband 112 m This 1s a layout of the CR125 HPP negda Rectangular guides position the exhaust valves that slide in and out to vary the exhaust port If the the piston The 1998 model piston The 1998 model vai EXHAUST WAIVE TIPS AND TUNIN Alth ess hgial principles the implementation ferent with each manufacturer and each type has irs own flaws and fixes The lise below gives you tips on how to install xhaust valves use the same and service the most common exhaust valves as well as some tuning tips HONDA HPP Hondas HPP starred as a butterfly operated canister mounted beween the cylinder and pipe Lt served ro conrrol the volume and length of the exhaust pipe It had little effect on the power and most after marker pipes eliminated the canister system The butterfly was prone to carbon seizure and required frequent maince nance The next generation HPP was used on the 1986 co 1991 CR250 This system featured two sliding valves that operated within close proximiry of the piston varied the exhaust pore time area in accordance with rpm The square valves moved and effectively off the ends of the valves the valves could fall into the bore and hit faped guid
64. k id soft tools such as brake cleang Gan locking agent pene AA scal grease and gasker sealer the engine while you work on ic Make an open square box from wood blocks A universal box for any engine can be made from 2x4 in blocks with the dimensions of the box being 5x10 in CC Specialry makes a ball vise for 100 thar is convenienc if you plan to rebuild engines frequendy To perma nendy remove the cempracion to use steel hammers when rebuilding engines buy a plastic mallet Snap on makes a These are the basic tools that you ll need to rebuild the ower end of a typical dirt bike engine Starting from the left this is a crankshaft instaltation tool that draws the crankshaft into the left case half The three point tool in the top center is a crankcase splitting tool Just below that is a wedge Lool thal prevents the crankshaft from slipping out of parallel alignment when pressed into the right case half The tools are available from Kawasaki and fit all lwo and four stroke dirt bikes The small device in the top righ s a flywheel extractor The Jong T shaped tool is a piston pin extractor CRANKSHAFT REPAIR Once the rod is pressed into one crank half install the rest of the rod assembly including the bearing N washers Place a machinist s square alongside the crank halves to align the free half before pr N e rod At the final stages of pressing the crank together insert a feeler gauge with the minimum ri fn ice
65. kcase and in the cylinder bore ar the same rime A two stroke engi completes a power cycle in only degrees of crankshaft rocarion c ue to a four stroke engine which tequires 720 degrees of crankshal un to complere one power Two srroke engines aren t as el Re four stroke they don t retain engines ineanin as much air a o in through the intake as the air is losr out the exhaust pipe If a two stroke engine could retai the same percentage of air ic would be twice as powerful as a four stroke engine because it produces twice as many power strokes in the same number of crankshaft revolutions The following explains the basic operation of che two stroke engine 1 Starting with the piston ac cop dead center TDC 0 degrees ignirion occurs and the gases in the combus 132 CHAPTER EIGHT tion chamber expand and push down the piston This pressurizes the crankcase causing the reed valve co close Ac abour 90 degrees after TDC rhe exhaust port opens ending the power stroke A pressure wave of hot expanding gases flows down che exhaust pipe The blow down phase stares and will cnd when the transfer ports open The pressure in the cylinder musr blow down to below the pressure in rhe crankcase in order for che unburned mixture gases co flow out the transfer ports during the scaveuging phase Now the transfer ports arc uncovered at about 120 degrees after TDC The scavenging phase has begun meaning tha
66. ked questions on these items and their effects on engine performance What is a long rod kit A long rod kit consists of a longer connecting rod and all the special pares that must be used with ic A longer connecting rod only changes the rod ratio between che stroke length and the cod s cener co center length Long rod kits do not change the displacement of the cylinder only che crankcase How does a long rod kit improve the powerband The longer rod allows the piston to dwell at TDC longer for a greater pressure tise Before the piston opens the exhaust in the middle of the stroke the longer rod will have greater leverage Near BDC the piston travels faster causing greater intake velocity on piston port engines and greater reed peral life and mass flow on reed valve intake systems How would the porting change with a longer dwell rime The operating speed is about 8 000 to 11 000 rpm The ee he the intake transfers and exhausta change even if the cylinder an at the same distance is he longer rod Why aC es in stock form have long rods wwtvork on all 125 ce hikes ras engine designers have to a number of factors when ig an cagine for a particular applica For example a rod ratio of 4 1 is considered ideal for a road racer while 3 75 1 is bewer for MX On small engines the greater the rod ratio the lower che primary compression ratio MX engines need a primary compression ratio of about 1 8 1 in order to have
67. l YAMAHA POWERVALVE Yamaha was che first motorcycle manu facturer to adapr exhaust valves co two stroke motorcycle engines Yamaha simple design uses a cylindrical valve thar rotates 1 4 turn to vary the height of the exhaust port and requires litle maintenance I he system was used on the YZ250 from 1982 co 1998 and on the YZ 125 from 1983 to 1993 Occasionally you ll need to replace the seals and O rings to prevent exhaust oil from drooling out of the side of the cylinder In 1989 Yamaha added a stop place to limit the travel of the power valve primarily so mechanics couldn t instal che valve in the wrong position This is view Irom the left side 125s and on all 250s 300s Scribe marks show sin s al linder of the KTM exhaust valve system used on the 1986 to 1997 380s The main shaft has a gear plate that turns the side drum valves t The stop plate with the two Allen bolts controls how far the main exhaust TOC In the KTM manual the stop plate is referred to as the 2 dimension M you valve closes in dista turn it too far the On will contact the piston and damage it The stop plate loosens and the valve breaks the ri gind the rahe 1 mm to prevent this problem The stop plate is located on the left side ae used ewo oval shaped wedge of the cylinder The valve has a small KS thar bumps up against the stop plate t limit the fully opened and closedyghsi tions of the valve
68. nd midrange power ave plenty of power down los fy midrange and a flar top e1 ross and enduro riders favor Na power Ir is easy to use and We rider confidence Most amp faster times and have more KS more low end and midrange ith tuning you can change your compressed and just before tie piston reaches TDC che ignici ororcycle s powerband to somewhere system discharges a spark caysi process all over again HOW TO CHOOSI POWERBAND By making chai gine compo nents nea japanese ditt bike has the porengial for two types of power The engine cah be cuned for midrange and high rpm power or for low end and midrange power The midrange and high rpm bike will have lirtle or no low end hir explosively in che midrange and have an abundance of top end power that can be over revved This kind of power can pur you our front in the straights but it is barder to control and will tire out the rider more quickly berween onc of these extremes Only a few riders use the extremes Professionals on ourdoor cracks especially 125 cc European Grand Prix bikes use engines that are almosr all high cpm power I hese machines are extremely fast and require highly ralented professionals to make the most of them Enduro riders in extremely slippery technical conditions use bikes tuned for lors of low end Trials riders use bikes thar are cuned for nothing but low end Generally speaking the higher the powerband peak hoth in
69. nder The L mark denotes che low speed position of the valve and the H denotes the high speed position To check the HPP state the engine Ac idle the valve should align with che L mark Then rev the engine the valve should align with the H mark If che angle of the mark on che valve is slightly off the valve probably needs to be de carboned The system is very easy to disassemble and can only fit cogether on thar procedure one obvious way so won t waste zO D There arc some aftecmarkec pi adjust the performance of chi for different cypes of dirt Wik Racing in England mal the right side valve Bayer Ye serves to add volume and le the tesonaror pate of the wets Tis is especially suited for x riding where a smooth waon co the midrange is impos Br better traction ESR dG anders Racing in California fe replacement HPP system that bots che valves wide open The center Kem valve is thinner which enables tuners to raise the exhaust port The FSR system is primarily used for ditt track or kare applications where low end power is of no consequence Whenever the cylinder is installed on the bouom end after top end rebuilding the valves need co be put in the closed position Otherwise the HPP cam spindle char connects the actuator in the cases to the cylinder will get damaged when you tighten down the cylinder Thar will also make the valves inoperable Always check che HPP v
70. ng business 1 use a Dremel model 732 It has changeable tool handles in straighe nd right angle configurations There are hundreds of tool bits varying in shape size grit and material from felt to diamond The popular uses of grinding and polishing include cylinder porting cleaning carbon from manifolds and pipes leveling rhe wear grooves on clutch baskets slicing the rusted bearing cages of swingarms and linkage polishing carbon from power valves and removing the molded rubber cars of two stroke intake manifolds for V Force reed cage installation Cylinder porting tool bits differ greatly for uses on nickel plated and cast iron Jined cylinders Plared cylinders are harder and more brittle It is a common mistake to chip the plating at a ports edge which can lead to ring snagging and bore failure Stones or diamond tooling cut through the hard nickel placing where traditional fluted carbide rool bits can take over the porting rasks Vircually any tool bits can be used on cast iron lined cylinders hecause the material is much softer The biggest supplier of porring tools in the motorcycle industry is CC PERFORMANCE MACHINING TOOLS AND TECHNIQUES Portable ganding tools have cable drven and foot operated electric motors with quick change tool capability They are used most often for cylinder and head porting tasks Specio tn Lawreniceburg Tennesee They sell everything from electric nke air
71. nges and the frequency of jumps and turns on a racetrack Low to midrange powerbands work well on soils such as mud and sand Tight tracks with lots of off camber or difficult corners wil favor low end to midrange power Smoother broader powerhands work well for enduro or trai riding over a variety of terrain and soil conditions Midrange to top end powerbands work best on terrain wich loamy soil long fast uphills and fast sweeping turns MAINTENANCE PRACTICES Generally speaking powerbands designed for low co midrange require less engine maintenance than powerbands designed 133 TWO STROKE ENGINES bos Le a TUNING FOR SPECIFIC POWERBANDS This chart is designed to give you some general guidelines on different powerbands and the changes required to the individual engine components For specific recommendations on your model bike refer to the chapters on tuning tips COMPONENT Cylinder Head LOW TO MIDRANGE 9 5 1 compression ratio squish band 60 percent of bore area MIDRANGE AND HIGH RPM 8 1 compression ratio 40 percent squish COMPONENT Cylinder Ports LOW TO MIDRANGE Exhaust port 90 ATDC transfer ports 118 ATDC MIDRANGE AND HIGH RPM Exhaust port 84 ATDC transfer ports 116 ATDC COMPONENT Reeds A LOW TO MIDRANGE Dual stage or 0 4 mm fiberglass petals KG COMPROMISE Thick carbon fiber petals MIDRANGE AND HIGH RPM Q 30 degree valve K COMPONENT Carburetor O LOW TO MIDRANGE Sm ljer
72. of exhaust valve systems The first generation design uses a variable volume chamber mounted to the head pipe to change the tuned length of the head pipe increase A butterfly valve separates the surge chamber and the head pipe Ac low rpm the valve is open to allow the pres sure waves in the pipe to travel into the surge chamber effectively lengthening the pipe and reducing the pressure wave s magnitude when ic returns to the exhaust port This design was primitive and nor very effective on 125 cc dirt bikes Honda and Suzuki used this type of exhaust valve system in the mid co lare 1980s The second generation design features valves thar control the effective stroke and the rime area of the exhaust port These valves are fitred to the sub exhaust ports and the main exhaust port the main exhause port valves operate within close proximity to the piston ro control the effective stroke of the engine The effective stroke is defined as the time from TDC to when the exhaust port opens Ac low rpm the engine needs a long effective stroke which results in a high compression ratio At high cpm the engine needs 111 TWO STROKE TOP END REBUILDING shorter effective stroke longer exhaust duration greacer time area and a lower compression ratio Yamaha used this system starting in 1982 on the YZ250 Honda s HPP system is similar and was used on the 1986 co 1992 CR250 and 1990 to currenc madel CR125 The third gene
73. omBessidh rario will also depend on whetkePor not the engine is quienes h exhaust valves and the condjidyFof the valves When the exha are in the closed position Sian ratio will be greater iF the valves are carbon seized in rhe open position The difference may yield a pressure reading of 25 psi The quality of compression testers varies greatly The main ching char a compression tester can identify is a change in condi tion Whenever you rebuild the top end take a compression pressure reading and mark ir down When the pressure changes 20 percent check the condition of the piston and rings Pistons usually last ewice as long as rings CRANKCASE PRESSURE TESTING The crankcase of a two stroke engine is sealed off from che rranny Irs impor tane thar the two crankshaft seals are in oprimum condition One side of the crankshaft uses a dry seal and the orher a wer seal The dry seal runs on the magnero side and che wet seal runs in oil on che tranny side When the dry seal wears the crankcase sucks in hot air causing che ayy overheat the ngii hen che wer seal se sucks in cranny il causigig the engine co run rich and eveni fec foul the spark plug kcase pressure test involves of a vacuum pump with spark gplug adapter and rubber plugs to block off che intake and exhaust manifolds of the cylinder The piston must be posi tioned ar BDC co allow che cransfer ports co be wide open linking the bore and the crankc
74. on it caused 2 shock when che valve was jo the wave thar impeded the ourgoing exhaust flow Grinding the edge smooth reduced the low end power bur helped improve top end In 1997 Suzuki This s the power valve from the 1999 to 2005 YZ250 It s a two stage system mounted to a single drive shaft redesigned the cenrer valve choosing where the center valve opens first and lhe sub exhaust valves open later based on the ramp angle of the dave steel and splitting the valve into two lug The sub exhaust valve cam levers aren t marked left and right so it s common for people to install them h makes the powerband run flal It s also common for the mounting plate for the center valve to crack allowing the valve to contact the piston sections a major and minor valve backward Suzuki also added a two stage spring 120 EXHAUST VALVE TIPS AND TUNING system With some simple grinding to march the valve co the exhaust port when fully open this setup was a winner Suzuki chose to redesign the 1997 system design for che 1998 2000 models The thought was thar the steel valve damaged the valve pocket in the cylinder although simply extending the nickel silicon carbide bore material the valve pocket would have solved the problem In 2001 2004 models Suzuki added many more successive designs of moving parts such as linkages and pins Ir a very into difticule system to assemble and requires a factory service manua
75. or high rpm engines Reed valves with four pecals are used for dirt bikes rhat need strong low end and midrange power Three other factors must be considered when choosing a reed valve the angle of the reed valve the type of reed material and che petal thickness The ewo common reed valve angles are 30 and 45 degrees The 30 degree valve is designed for low end ro midrange power and the 45 degtee valve is designed for high rpm power Two types of reed petal materials are commonly used carbon fiber and X fiberglass Carbon fiber reeds are light weight bur relatively stiff spring tension and are designed to resist fluttering ac high rpm Fiberglass ceeds have sclacively low spring tension to instantly respond ta pressure changes in the crankcase however che low spring tension makes them Autrer ac high rpm thereby limiting the amount of pare Fib ge reed petals are good for low imideange powerbands and carb r reeds are berrer for Nok Ss les Regarding longevity iene is rend to splic whereas carboytfiber tceds tend to chip Some eos reeds such as the Boye aS cage reeds have a large iat greed with a smaller thinner reed muned on top This setup idens che rpm range where the reed re flows best The thin reeds respond to low rpm and low frequency pressure pulses The thick reeds respond to higher pressure pulses and resist ut tering ar high rpm I he Boyesen RAD valve is different than a craditional re
76. pnng may snap loose upper rack and lower rack simultane ously Take care and be patient You may have to wiggle the wedge valve yoke to get everything ro fall into place Never hammer the drum valves Push down the right drum valve and valve systems has to do with the actuating lever spring On the ft that the valves are linked to If you remove the actualing lever the The spring ends should always be parallel in order for the valves to move instal che idler gear Now install che bushings and check the system The valves will bind and stick if you try to move the valves without the bushings installed or if the cylinder is facing upside down Test che KIPS in this EXHAUST VALVE TIPS AND TUNING way pull the rack ourward until it stops and chen look through the exhaust pore from the pipe side The valves should be in the full open posi tion On cylinders where the base has been turned down more than 0 010 in the drum valve bushings will also need to be turned down co prevent the valves from binding when the cylinder is tightened SUZUKI ATEV Suzuki first used exhaust valves in 1985 using a drum valve thar uncovered a cavity in the head or cylinder to add volume and length to che exhaust pipe strictly at low rpm In 1987 they employed a system thar featured two large valves rhat had multiple functions The system was used on the 1989 2000 RM80 1987 2000 RM125 and 198
77. purchased from Goodson for 25 For offset heads you ll need to fasten and space the heads on bolts and pins on a face plate To curn cylinders expanding mandrels are the best setup Lhe mandrels adjustable sleeve clamps on the cylinder bore Preci selection of expanding mandrels buc inandrel kits can cost 200 co 1 000 The most cost effective way to buy ion Devices has the biggest cutting tools is to purchase a kic with five tool bit holders and replaceable tips Considering chat most machining tasks on dirt bikes will be performed on Honing machines are either portable or stationary The hone on the left s a portable lool that can be riven with a drill Sunnen makes stationary honing machines with a wide variety of mandrels and stones Diamond stones are required to hone modem plated cylinders aluminum paris the TiN coated rips work best because they resist buildup of aluminum on the cutting tip Milling A vertical mill is like a lathe turned up on end The main difference is that on a lathe the cutting cool is stationary and a mill spins the roo with the work piece mounted stationary The uses for a mill include installing flywheel weights that use three Allen bolts baring crankcases removing broken bolts resurfacing gasket surfaces such as crankcascs boring cylin ders and back cutcing tranny gears For my business 1 chose a combina tion machine that cnahles laching and 1
78. r be susceprible to pressure blowouts into the clip groove Place one clip in the groove before installing the piston on the connecting rod It s easier to install a clip with the piston in your hand than on the rod There 11 TIPS FOR REBUILDING A TWO STROKE TOP END 1 Before you begin power wash the engine and the test of the vehicle co is also less chance that you ll drop the circlip in the crankcases TROUBLESHOOTING PISTON FAILURES 6 Always install che rings on che piston with the markings facing up Coat the rings with pre mix oil so chey slide in che groove when trying to install che piston in the cylinder 7 Always install che piston on the connecting rod with the artow on the piston crown facing toward che exhaust port 8 Ihe traditional way ro assemble che cop end is to install che piston assembly on the connecting rod comptess the tings aod slide the cylinder over the piston That can be difficult with larger bore cylinders or if you re working by yourself Try this method instead Install one citclip in the piston place the piston in the cylinder with che pin hole exposed install the piston pin through one side of the piston position the cylinder over the connecting rod push the piston pin though until it borroms against the circlip and install the other circlip Ic only cakes two hands to install the top end this way and there is less chaoce thar you ll damage the rings by owisting the cyl
79. r spark plug range too lean TROUBLESHOOTING PISTON FAILURES o 9 Multi Point Seizure This piscon has multiple vertical seizure marks around the circumference This cylinder was bored to a diameter that was too small for the piston As soon as the engine started and the piston starred irs thermal expansion the piston pressed up agaiost the cylinder walls and seized The optimum piston to cylinder wall clear ances for different types of cylinders vary greatly For example a 50 cc composite placed cylinder can use a piscon to cylinder wall clearance of 0 0015 in whereas a 1200 cc steel sleeved cylinder snowmohile set up for grass drags will need 0 0055 ro 0 0075 in For the best recommendation on the optimum 10 Intake Side Seizure This piston seized on the intake side This is very uncommon and is caused by only one thing loss of lubrication There are three possible causes for loss of lubrication no pre mix oil separation of the fuel and pre mix oil in che fuel rank water passing through the air filter and washing the oil film off the piston skirt 11 Composin Moding Mosc coi cylinders used on motor cydes ahd stowmobiles have composite Inders The composite material t pi ronita yliider de ran e for your fs fo anyala abide parcels engine look co the specs that come pacity T Aotcio plating procis enables he aged with the piston or consult O factory service manual ilicon carbid
80. ration design of exhaust valve systems arrempts to change the exhaust porr velocity effective stroke exhause gas temperature and compres sion wave pressure Yamaha and Suzuki starred using rhese systems on their 125s in 1995 Boch companies employed a venting system to the outside atmos phere his is very complex because they are attempting to affect the temperature and pressure of the tecuning compres sion wave to synchronize it with the piston speed Two oval wedge valves that enter the exhaust port at a 45 degree angle control che exhaust gas velocity and the effective seroke The wedge valves partially block the exhaust port thereby boosting the gas velocity Kawasaki s KIPS system uses wedge valves in the main exhaust port ro control the effective stroke drum valves in che sub exhaust ports co control the ume area and a surge chamber co absorb the excess comtpression wave pressure at low rpm A centrifugal governor mechanjsr opens and closes the exhaust valves governor is mounted under th side cover and is gear drives crankshaft As the engine x creases the governor spins ing the angular momentum balls encased in e balls fit into a led ramp and cup arrangemen Gr ples tension 00 the sre en the momentum of the sce e four steel nor I he steel alls overcomes the spring s tension che balls force their way up the angled ramp A spool atcached to the tamp enables it to change its linear posi tion w
81. rtual lwo Seroke or Dynomacion There are a lot of things going on in a two stroke engine When you change one thing like the stroke several other things change too fo herter or worse a WHAT HAPPENS WHEN A go These are the things that a when the stroke is change The displacement i om The pore timing Kon The ignicion ging vances The c ion rario of che combusttgh chamber and crankcase increase The reed valve timing advances and the recd lift increases The piston speed is greater at any given rpm PRUNE wa 6 xN The maximum piston speed is reached ata lower rpm 8 9 10 The bore to stroke rario is altered The rod bearing wear accelerates The rod ratio decreases WHAT THINGS MUST BE ACCOUNTED FOR The cylinder must be shimmed up or the head s squish band must be machined to compensate for the increase io stroke 1 The port time area must be increased to compensate for the stroke and displacement change The ignition timing may need co be retarded The combustion chamber in che head must be enlarged for greater volume The connecting rod bearing and piston pin bearing must be changed more often The crankcase diameter may need to be increased for rod clearance due to the greater offset of the rod FOUR WAYS TO STROKE A CRANK There are four popular ways co chan the stroke of a crank x 1 Manufacture new crank ery ch the dimensions built in 2 TIG wel
82. s K and vintage Spanish cranks Alhol some aspects of lower end rebuildis very specific to a particular mode e this section gives you an amp i general process of the THE RIGHT TO Engine rebuildin without the gi Some guys try to use caveman merhod big hn el chisels They usually end up doing Some stupid thing that mins expensive engine components To properly rebuild che lower end you will need the following tools from arly impossible the manufacturer a service manual for torque specs and disassembly assembly rechniques specific to your model engine a flywheel puller a cluch hub holder a crankcase splitting tool and a crank shaft installation cool but PII show 124 CHAPTER SEVEN you techniques for remoying the clutch and installing che crank so you can save money on those tools You will also need an air or electric powered impact wrench to remove the nuts that retain the flywheel chuch counrershatt sprocket and primary gear a parts washer with solvent to clean the engine parts a hydraulic press to remove install che bearings a hammer will damage them a propane torch t and expand the aluminum cri remove or install che bea they have an interfere that the bearing is a erer chan che hi digital caliper fecause meaning ger diam ic fits into a ure certain engine parts and ets co the minimum a listed in the service nal variety of wrenches and soc
83. s with a rapered panheg n bolr Take care when handli on lever on the left side of iN valve Before you loosen Allen bolts scribe a line D the position of the plate Ai to che gear plate It is possible co st che srop plate so far that che valve rorares past the fully closed position and contacts the piston That will destroy che piston The 1998 and newer KTMs have an exhaust valve system rhat makes use of che resonator concept The system is so complex thar you would need the in order to factory service manual service ic 123 Rebuilding the lower end of a two stroke engine is the procedure that is most often put off until nexe race inonth season When you stare hearing the engine make a strange knocking sound it s time to shur ic off and tear ic down Donc pin the throule wide open and hope ir will just go away The normal service incerval for lower end rebuilding is once a year on engines under 200 cc and once every ewo to three years for 250 and larger engines While rebuilding the lower end you should replace the ball bearings thar support the crankshaft and the transmission shafts plus rhe rubber seals In mosr cases the crankshaft will need to have a new connecting rod pin bearing and thrust washers installed Some manufacturers Honda don t sell parts for their crankshafis only che entire part However there are companies that offer high quality replacement parts Hor Rods to rebuild modern Japanese cra
84. seals bored the cylinder and fitced a new piston 110 15 Bridge Point This piston has a deep wear mark in line with the exhaust bridge Two diferent things can cause a problem like this he most common problem is that the rig side crankshaft seal is leaking cays cranny oil to enter the bore F rot charge of rhe friction pres exhaust bridge and the oi an attraction The oily an abrasive media amp ar the exhaust br letare the wear other common problem is a proper relief clear ance on the Wuse heidge When a cylinder Peps or overbored che exhaus Nge muse be ground for extra tales over the bore size because the Wt bridge gets hotter than other areas of the cylinder Ihe extra relief learance compensates for expansion The normal relief clearance is 0 001 co 0 003 in Generally speaking the wider the exhaust port in telation 10 the bore size and the narrower the bridge the less relief needed Too much bridge relict will cause the rings to flex the exhaust port and break r 16 Piston Bounce Do you norice the distinct circular line located near the outside of the piston Jt was caused by contact berween the piston and che cylinder head s squish band ar TDC top dead center There are several causes for this problem ranging from worn connecting rod bearings to an improperly modified cylinder head The foun clearance beeween the pisto Aina at TDC is 0 040
85. t pipe Notice how carbon buildup has reduced the diameter of the pipe which can make the bike lose power and run hotter if your bike ever breaks a piston power wash the pipe to flush out the debns Some big bore kits have piste f than the hole i the crankcases Clear Ing is necessary and can be performed Kye taping and the aid of a vacuum to There ire three cypes of plated cylinders Kawasaki Flectrofusion hard chrome and nickel silicon carbide Vhere are several variations of the nickel silicon carbide process but che mast cornmon trade Nikasil The nickel based processes have many advantages over hard chrome Eleccrofusion and name is sleeving Nickel auracis oil and is an excellent carrier materjal for silicon 102 Whenever the cylinder is overbored ong Meo valve clearance to the bore These valves protrude too far into the bore and need to by ery piston clearance of 1 mm carbide particlesdy wett resistane mate rial that carriey eftoad of the piston The mater N arrai right on to the alum a inder for the optimum iency Nickel can be honed wirliamood stones which leave distinc chery peak and valley scratches in the cyTinder wall that retain oil and provide a certain bearing ratio between the running surfaces of the bore les possible ro rebuild a placed cylinder by fiting it with a sleeve However you can expect co pay more for bore maintenance over the lif
86. t the unburned mixture gases are flowing out of che transfers and merging together to form a loop The gases travel up the back side of the cylinder and loop around in the cylinder head ro scavenge our che burnr mixture gases from the previous power stroke It is critical that che burnr gases are scavenged from che combustion chamber to make rooin for as much unburned gas as possible The more unburned gases you can squeeze inro the combustion chamber the more power the engine will produce Now che loop of unburned mixcure gases has traveled into the exhaust pipe s header section Now the crankshaft has roraced past bottom dead center BDC 180 degrees and the piston is on the upstroke The compression wave reflected from the exhaust HOW TO CHOOSE A POWERBAND Case reed engines can benefit from crankcase porting and cylinder matching more so than cylinder reed engines Case porting is a metal finishing task that serves to match the crankcase and cylinder transfer ports and polish surface imperfections pipe packs rhe unburned gases back in chrough the exhause port as the piston closes off the port to scare the compression phase In the crankcase the pressure is below atmospheric producing a vacuum and a fresh charge of unburned mixture gases flows through the teed valve into the crankcase The unburned mixture gases are Expert outdoor pagi use engines tuned for hi Wer An engine cunedSfor Yow cnd a
87. tate rhe rings u one side of che locating pis pnt squeeze the rings wi Middle finger and ins AE will Jeave your other hand free position the cylinder 6 There are two methods used to assemble the top end The first merhod is to attach the piston co the connecting rod and lower che cylinder on to the piston assembly The second method is to install the piston assembly into the cylinder and lower the cylinder and piston on to the connecting rod The second method ip overcoming the force of the spring is easier bur involves pinning the piston and installing one circlip with a minimum amount of free space 7 Take care ro align che exhaust valve conrrol mechanism as the cylinder bolted to the crankcases x GASKET HYGIENE The oven cleaner you a the cylinders will help logo Me old gasker material enongh coptengve ic Carefully scrape the gasl ith a gasket scraper Never use a fl driver to remove the old prienti the aluminum surfaces of the eylinder and crankcases are ly If these surfaces are gouged amp engine they should be draw filed Ker prevent air or coolant leaks Never reuse paper gaskets always replace them with new gaskets and spray sealer on che paper gaskers so they will scal bercer and will be easier to remove the next time The new sryle stecl gaskets can be cleaned and reused a few times bur you ll need ro spray the gasket with a sealer such as Permatex Spray A Gaskec or copper coat KEEP A
88. tem system is very 116 lt carbon seized the steel teeth on che rack shear off rhe aluminum This is a typical KIPS system for the tude 2000 KX250 As you can see there main center valves and the drun anfia gi Da moving parts The top a heel racks have to be synchronige Jthrough the left drum valve has two drive gears molded 3 The drum valves are made of imum When the drum valve teeth on the drum valve rendering the drum valve inoperable Check the condition of the gear reeth every time you do a rop end service because if one gear fails the whole sysrem runs out of sync On the lare model 80 cc and125 cc KXs the KIPS is relatively simple relying on a wedge valve and flapper The system is self scraping so it requires little maintenance In the first year of operation 1998 the KIPS system was plagued with failures such as the pin breaking on rhe flapper the valve receding inro the cylinder and contacting the piston and over excension of the valve causing cock and jam Pro Circuit made an aftermarket valve cover with a full stop that prevented over extension and in 1999 Kawasaki changed the wedge valve and flapper xe 1992 to 1997 KX125 1995 to 2000 KDX200 and 1993 to a moving parts The two racks must be timed with gears Lo link the is valve system performs well but il requites a lot of maintenance design for more rigidity which solved all the reliabilicy problems Ihe drum valves on rhe 19
89. the form of engine management systems that control the ignition system power jet exhaust valves and resonator temperature and ae lled in the three jaw chuck of a lathe to facilitate tuning changes pressure Right now we re seeing new products such as programmable or switchable ignition boxes In the near future we ll sec a continual stream of products that will culminate in a direct injected two stroke engine wich a management s that coordinates system the ignition intake exhaust and ulti mately the powerband with just a tweak of the chumb The Optimum GP Conrrol package for the Suzuki RM250 is a kit that KX250 catburetor with pumper and throtle includes a handlebar switcb position sensor and a computer thar fies under the seat on a hinged place Tbe unit coordinates the ignition system with the fuel pump The handlebar switch offers 42 carb s positions and the jetting and ignition can be adjusted on the fly There is also a sepa rate bole shot switch that provides smooth power delivery and better trac PERFORMANCE MACHINING TOOLS AND TECHNIQUES This cylinder s fastened on an expanding mandrel and mounted in a lathe The cylinder dase 1s being turned Q down to reduce the ports duration retard the liming and increase the compression ratio This machining gpl ation serves to improve the low to midrange power of a dirt bike rion You can program the control package with
90. ts bur che profile that best suits both steel and placed cylinders is aluminum oxide or silicon carbide 240 to 360 grit A hall hone cannot remove material from the cylinder bore especially on hard nickel plated bores However a ball hone can polish down the peaks of the original hone scratches and increase che bearing ratio In other words the piston will be touching a greater percentage of the bore Sometimes that BIG BORE KITS A centrifugal governor mechanism converts rotary motion from the crankshaft and turns it into linear motion to vary the exhaust port s effective stroke matched to rpm The four steel balls travel in channels on a ramp The higher the rpm the farther the balls travel up the ramg makes the piston wear quicker bur if you have ro ball hone the bore to remove scratches its a compromise Never use a spring loaded finger hone on a cwo stroke cylinder The sharp edges of the scone will snag che port edges and most likely damage the hone and the cylinder TOP END ASSEMBLY 1 Insrall one of the circlips in the piston with che opening facing away in the 6 or 12 o clock position 2 Grease the cylinder base alignment pins 3 Set the exhaust valves in the closed position 4 On cylinders wich reed valves leave the intake porr open because yo will need to reach in through t port to push the pisron ring Ads back in place 5 The best way to slip d on into the borom of th ler is to ro
91. um color buc QX dice bikes and covers rips on how to some oils such as Yamalube R hav additives thar prevent carbon f forming on the piston crowny N 2 Front edge The two bl ocs align with the sub exha ks and indicate exhaust b1Rbadk When rhe sub cxhaust por alvc are closed If the exhaust were carbon seized and RA dosed the carbon patrerns Be much larger 3 Rear The rwo small black spots are above the ring alignment pins The pots show pressure leakage ibly from excessive ring end gap which enables the combustion gas co escape past the ring gap and leave a carbon trail in its wake 4 Rear band Ihe intake side of the squish band has some light brown patterns That is the appropriate color for the center pattern when the carb jetting is righe If che color were dark brown it would mean thar the engine is running on the rich side or thar the engine doesn t run ac the right cemperature 5 Center The center pattern shape can explain how the engine is cunning This pattern is shaped like a heart showing the flow patterns chrough the transfer ports Notice rhe jeft side is slightly larger which indicates thac che flow rate of the right side transfers is greater chan the left side Thar means that the arca and timing time exhaust valve systems How Exhaust Valves Work An exhaust valve system is designed to the engines low end and midrange power here are three different designs
92. want from your engine What kind of riding do you do Whar level of rider are you How much money do you have to TUNING GUIDE TO PERFORMANCE MODIFICATIONS and transfers thar alters che timing area size and angles of the ports to adjust the powerband to better suit the riders demands For example a veteran trail ng an RM250 in the Rocky Mountain region of the United States will need to adjust the powerband for more low end power because of the steep rider rid hillclimbs and the low aig density of higher altitudes I he onl k to deter mine what changes taakeengine will be necessary is to oY calculate the stock engine s sphgifichti The mosveritical measurement is che port time Ka calculation of a port ope A3 and timing in relation to the Wigplgcemrent of the engine and the rip Wepeiteniond anes kugi whit Kaus work best for different purposes polsevald v rsi eiid t Por erato ple In general if a tuner wants to adjust the engines powerband for The cylinder on the left 1s ported and the cylinder on the right is stack Porting is a metal mad ration more low co midrange he will do the that includes general smoothing and matching of the ports with port ining changes mat itersection following two things of the port windows and the cylinder bore 1 Turn down the cylinder base on a gt lathe to increase the effective stroke distance from TDC to exhaust port spend Remember thar
93. weld the cones petma nently into place Finally they cut and rotate all the cones so the pipe forms a C shape Then they cut off the header flange and the stinger From there the pipe goes to the die maker who makes the aluminum casting of the pipe that becomes the press die lorgue and Rev Pipes Pipes are stamped from sheet metal into a C shaped partern Press dies have a limited life span rated in che number of parts stamped Ic isn t that they crumble to pieces bur they do slowly wear When the press die wears it changes the dimensions of a part Dies start our larger than the target size and weat The parts ac the beginning and end of the dies life are different sizes Pipes are fabricated by welding two C shaped pipe shells together chen che sections are cut rotated and welded again Fvery time the pipe is cut on the band saw the length is reduced by the width of the saw blade and che amount of material thac is sanded off the cut edges Multiply that by che number of cuts and a pipe can shift its tuning range prety quickly especially if an overzealous worker sands coo much here and there Manufacturers rate pipes by their length A long pipe is a torque and a short pipe is a rev The engine manu facturers do the same thing with pistons and cylinders with the markings A and B The label is a dimensional represen tation of a finished manufactured part Some manufacturers like FME don t
94. xpert before tack ling a big bore kit lo get the most from an overbored engine you need to make sure the carburetion exhaust porting and riming are all adjusted co suit the larger bore There are several companies specializing in alternative displacement kits involving both overbores and crankshaft stroking PORT TIME AREA Porr time area refers co che size and flow range of che intake and exhaust ports relative to rpm The ports enter the cylinder bore at angles When the cylinder is overbored the transfer ports become lower and wider The same thing happens to the exhaust port This effectively retards rhe port timing and reduces the toral degrees of duration When the displacement of che engine increases so docs the demand for more port time area 105 TWO STROKE TOP END REBUILDING If you just overbored and plated a cylinder it would have much more low end power chan stock bur the top end power would suffer Notmally tuners have to adjust the ports to suit the demands of the larger engine displace ment Ihe propet dimensions for the ports can be calculated using a compurer program from Two Stroke Racing TSR The program Port Time enables tuners with limited inath skills to run steings of formulas for determining the optimum dimen sions of the ports Generally speaking if the ports in the overbored cylinder were raised ro the same heights as the stock cylinder it would make the port timing

2-stroke top end rebuilding & performance

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2-stroke top end rebuilding &amp; performance

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2-stroke top end rebuilding & performance