HS40 tuning manual
Contents
1. BN 38 43 Lever A Pump TM 36 18 O Ring B 30 205 Spring A Pump FY 4633 Lever A Pump TM 36 17 Spring A P Adj M12F 46a Screw A P Adj MC 0316 Hexagon nut A P Adj N3 03 B CAP Vacuum Fitting N148 013 Spring Clip Vac fit BVK 28428 CAP A P Rubber TM 36 64 Rod A Pump TM 36 44 Plunger TM 36 60 Spring A Pump VM 14SC13 89 Plunger Starter VM 38 148 Hose 888 23026 Screw N V Holder VM 13 216 Screw Float Bowl C2 0412 B Starter Jet VM 17 1002 45 Packing Shaft Mk 28474 Ring A P Spring 204046 Cap Starter Cable VM 12 43 O Ring Starter N 138 198 Spring Starter VM 16 42 Guide Holder 640 12001 Plate Fuel Inlet FM 40 26 Inlet Fuel TM 40 27 O Ring needle valve KV 10 Choke Cable Assy 990 662 001 PART NUMBER 008 666 NO AVAILABLE STOCK2. making any alterations to the HS40 if any are needed at all read the section of this manual describing the various components and their functional range There is simply no point in attempting to tune any carburetor unless the engine is completely sound Valves and rings must seal properly the ignition timing must be correct and the spark plugs clean and gapped Some exhaust systems may also make carburetor tuning difficult For instance it is almost impossible to get smooth respon sive carburetion with straight and open pipes If you have and doubts about the condition of your engine tune and test it before beginning what could be a frustrating and unproductive effort to fix another problem with the carbure tor TUNING THE IDLE CIRCUIT PILOT SYSTEM The first step in tuning any carburetor is to get the idle circuit correctly adjusted And the first step in this procedure is to adjust the pilot screw position for best idle Mikuni sets the pilot screw at three turns open during assembly This is the position we have found to be right most of the time If the screw position has been altered gently bottom it and re open to three turns out Next ride the bike until the engine is at its normal operating temperature This may require several miles at highway speeds If you have an oil temperature gauge ride until the oil temperature is at or near 150 degrees With the machine vertical and the engine idling near 1 000 rpm adjust the p
3. pilot air jet has the reverse effect A larger pilot air jet leans the mixture and a smaller one richens it The pilot jet and pilot air jet have slightly different effects on mixture strengths These effects are discussed in the General Tuning Procedure portion of this manual The idle circuit can be adjusted by changing either the pilot or the pilot air jet A one size larger pilot jet will have nearly the same effect as a one size smaller pilot air jet It is generally easier to change the air jet since it is more accessible After changing either jet it is necessary to re adjust the pilot screw for best idle MAIN SYSTEM The main system dielivers fuel from 1 16 to full throttle The idle circuit delivers the mahority of the fuel near 1 16 throttle The main system becomes the important mixture control from about 1 4 throttle The main system has three tunable parts 1 NEEDLE JET controls mixture from 1 16 to approxi mately 1 4 throttle varies with needle position 2 JET NEEDLE controls mixture from 1 4 to 3 4 throttle 3 MAIN JET controls mixture from 3 4 to full throtle The jet needle has a constant diameter section and a tapered section The diameter of the needle and the inside diameter of the needle jet form an orifice through which all main system fuel must flow Until about 1 4 throttle the constant diameter sec tion of the needle is within the needle jet and main system fuel flow is controlle
4. to emerge from the mouth of the needle jet LEAN CONDITION If the needle jet is too small lean part throttle acceleration will be flat There may also be some detona tion during part throttle acceleration although this can be caused by other factors A lean needle jet will also result in a slow warm up If part throttle acceleration is flat install a one size larger needle jet and compare the performance If accelera tion is improved leave the larger jet in and take a fairly long ride at steady speeds to give the spark plugs time to color evenly Take a spark plug wrench with you and after a few miles at steady speeds stop and remove a plug for inspec tion Be careful as you stop not to operate the throttle The extra fuel from the accelerator pump can cause a false plug reading The body of a spark plug can be from light grey to brown to dark grey If the plug body is black and has a sooty appearance then the needle jet is probably too rich and a smaller one will need to be fitted RICH CONDITION While a black and sooty spark plug is a sure sign of richness there are others that are a bit more subtle If your engine responds crisply at low throttle when it is cold chances are the needle jet is one size larger than it needs to be Assuming of course that the idle circuit is correctly tuned and adjusted Poor fuel mileage is another sign of richness and because of the way most of us ride our Harleys that rich ness is usua
5. which it injects fuel can also be controlled with different sized pump nozzles Total flow volume depends upon the pump s beginning and end point adjustments STARTER SYSTEM Mikuni s starter system takes the place of the crude choke mechanism of the stock carburetor It is actually a small auxilliary carburetor designed to supply a rather rich mixture for starting purposes The starter system only works when the choke knob is pulled open and when the throttle is closed If the throttle is opened with the choke knob pulled out air essentially ceases to flow through the starter system and it stops delivering its rich mixture to the engine If the engine begins to load up while the starter system is engaged it may be cleared by opening the throt tle When the throttle is closed the starter system resumes operation The starter system has one replaceable compo nent the starter jet A larger starter jet makes the starter mixture richer and a smaller one makes makes it leaner The 55 jet fitted to the HS40 will suit most installations However if you live in a warm climate the alternate 45 jet may perform better DETAILS 1 Throttle return spring adjustment The throttle return spring has 3 pre load positions The HS40 is supplied with the spring in the stronger of the 3 positions You may wish to try a weaker spring position However in some installations the softer spring position may result in erratic return to idle 2 Fu
6. Mikuni HS40 Tuning Manual Exploded View and Parts List NEEDLE JET THROTTLE VALVE PILOT SYSTEM PILOT AIR JET PILOT FUEL JET PILOT FUEL SCREW Figure 1 Diagram showing the effective range of the each tuning component of the Mikuni HS40 carburetor Tuning Systems Description IDLE CIRCUIT PILOT SYSTEM The idle circuit supplies fuel at idle soeeds and has a major influence on fuel flow up to 1 4 throttle There are three tunable parts in the idle circuit 1 PILOT JET controls maximum fuel flow through the idle circuit 2 PILOT AIR JET controls the maximum amount of fuel that will flow through the pilot jet by allowing a higher smaller air jet or lower large air jet vacuum signal at the pilot jet 3 PILOT SCREW controls how much fuel is allowed to enter the carburetor venturi The pilot screw is used to control idle mixture Turn the screw out to richen the idle mixture Turn it in to lean the mixture The engine should have a smooth steady idle with the screw between 1 4 and 3 1 2 turns out from fully bottomed gently If the engine requires more than three turns out the pilot or pilot air jet may be too lean If it requires less thatn 1 4 turn it may be too rich As the throttle is opened the pilot screw s position becomes less important than the sizes of the pilot and pilot air jets A larger pilot jet richens the mixture from just off idle to 1 4 throttle A smaller one leans it A change in
7. d by the size of the needle jet Needle jets are available with different inside diameters A larger needle jet richens the mixture within its range of operation Notches at the top of the needle allow it to be raised or lowered Raising or lowering the needle deter mines the throttle setting at which the tapered part of the needle is raised out of the needle jet Main system fuel flow is controlled by the needle s taper from where it begins to lift out of the needle jet until about 3 4 throttle Raising or lowering the needle respec tively richens or leans the mixture from that point until the main jet becomes effective At about 3 4 throttle the orifice formed by the needle and needle jet becomes large enough that the size of the main jet begins to control fuel flow Until this point the main jet has no effect on mixture strength At full throttle the needle taper has little or no influence ACCELERATOR PUMP Mikuni s HS40 carburetor is fitted with an accelera tor pump The purpose of the pump is to inject fuel into the throat of the HS40 when the throttle is opened As the throttle is opened especially at low rpm air velocity through the carburetor drops and the mixture naturally becomes leaner Fuel from the pump maintains a more correct fuel air mixture until the air velocity returns to normal The accelerator pump can be adjusted to inject fuel into the throat of the carburetor over a wide range of throttle openings The rate at
8. e adjustor components HS40 SMOOTHBORE EXPLODED VIEW NO DESCRIPTION PART NO 01 Mixing body FM46555 0 02 E Ring BS32 126 03 Washer Needle clip 826 03002 04 _AHen Serew _ _ _ ___ 739 4360 05 Screw synch lock C5 0514 06 Gasket top cover TM 36 39 07 Top Cover 08 Lever Assy 09 Gasket funnel 10 Funnel 11 Screw T V Plate 12 Plate T V Lever 13 O Ring AP Nozzle N124 063 14 Pump Nozzle TM 36 43 15 Plug A P Nozzle TM 29 14 16 Washer T V Lever 17 E Ring T V Lever 18 Packing T V Lever 19 Clip T V Lever 20 Ring T V Lever 21 Plate T V Lever 23 Jet Needle J8 9DJY04 Throttle Valve CALL YOUR DIST O Ring Fuel Screw N 133 037 Washer Fuel Screw VM 12 33 Spring Fuel Screw N 133 206 O Ring Float Bowl 616 94021 Screw Pilot Fuel 604 26003 Float Chamber Body TM 40 12 Pilot Jet VM 28 486 17 5 Needle Jet 784 13002 Y6 O Ring Drain Plug VM 28 254 Main Jet N 100 604 165 Drain Plug TM 32 41 Float Assy 859 32011 Pin Float VM 30 160 Needle Valve Assy 786 36007 3 8 Main Air Jet Plugged BS30 97 Screw Crank Bracket C5 0512 B Shaft Throttle 70045004 Packing Ee Pilot Air Jet BS 30 97 1 1 Hexagon Nut N2 03 B Adjust Screw TM 36 48 Bracket Bell Crank TM 40 14 Pin Throttle Bell Crank N 138 019 Lever Assy Bell Crank TM 40 23 Spring Idle Adjust N150 025 Adjuster Idle T M40 77 Spring Throttle Return FAYt 4625 Screw Top Cover CW2 0408 Screw Air Funnel V36H33 Pin A P Locate
9. el float adjustment Fuel float level is critical to proper operation of any carburetor the HS40 is no exception Mikuni correctly adjusts the floats during assembly but they may be acciden tally bent out of adjustment during tuning or other handling If you have doubts check the float level and re adjust if needed A float level that is too high can make fine tuning of the idle circuit impossible One that is too low will have a similar effect on mid range tuning Figure 2 Diagram showing float level range and checking procedure Tuning Manual GENERAL TUNING PROCEDURES Your Mikuni is fitted with the tuning parts we found to work with a majority of engine tuning combinations How ever the tremendous number of differing exhaust systems and cams available for Harley engines make it impossible to accommodate all possible combinations with one carbure tor set up You will probably find that the HS40 will run perfectly on your engine without exchanging any parts But if it doesn t you may alter its tuning to suit your engine s needs by following this guide There are many more replaceable parts that affect tuning in the HS40 than in the stock Harley carburetor With these parts you can precisely tailor the HS40 s tuning to your engine s requirements throughout its rpm and throttle setting range Each tuning system is easy to modify and diag nose but only when you understand what each system does and how it works Before
10. have slightly different effects on mixture strength within the effective tuning range of the pilot system 0 0 to 1 4 throttle When you change the pilot jet it has a slightly greater effect on mixture strength at zero throttle than it does at 1 4 throttle On the other hand when you change the pilot air jet it has a slightly greater effect above 1 8 throttle that is does below that setting If your engine has slightly soft acceleration just as the throttle is raised from the idle position the size balance between the pilot jet and and pilot air jet may be incorrect If the softness is more pronounced whe the engine is at normal running temperature install a larger leaner pilot air jet If there is minor coughing or popping through the carburetor when the engine is cool install a smaller richer pilot air jet After you have changed the pilot air jet you will need to re adjust the mixture screw If the new mixture screw adjustment is outside the one to three turn range change the pilot jet If the mixture screw must be more than three turns out to get a correct idle install the next larger richer pilot jet However if less than one turn is required to get a good idle fit the next smaller pilot jet It is unlikely that changing the pilot jet will require changing the pilot air jet FIG 9 ACCELERATOR PUMP TUNING ADJUSTING SCREW 2 PLASTIC LEVER ADJUSTING SCREW 1 PUMP ROD Figure 3 Accelerator pump strok
11. ilot screw in one half turn at a time until the idle either slows or becomes irregular The pilot screw is now in too far and the idle mixture is too lean Pause for a few seconds at each half turn to allow the engine to settle down and give a clear indication of the mixture strength Now begin turning the screw out in half turn inter vals until the engine again slows or begins an irregular idle Count the turns between the too lean and too rich positions Set the pilot screw mid way between the too lean and too rich positions You may further refine the pilot screw position with riding experience but this setting will be very close to the correct idle mixture setting If you allow the engine to get too hot during the pilot screw adjustment procedure the resulting adjustment will probably be on the lean side of correct If you have a large fan use it while adjusting the idle mixture If you do not have one you may need to take time out for a short ride to cool the engine back to the normal temperature range Remember if best idle is achieved with the pilot screw less that 1 4 turn out the pilot jet is too large or the pilot air jet is too small One or the other will need to be changed On the other hand if the pilot jet screw must be more than three and a half turns out for best idle the pilot jet is too small or the pilot air jet is too large One or the other will need to be changed If the pilot screw s best idle position is
12. ives the highest rpm or speed at the second marker is the correct one This method is simple and com pletely accurate ACCELERATOR PUMP TUNING The beginning point of the stroke is adjusted with adjusting screw 1 on the white plastic pump lever To start the pump sooner back the screw out Turn the screw in to make the pump start its stroke at a larger throttle opening Most engines perform best if the pump begins its stroke as soon as the throttle is moved from the idle position The end point of the accelerator pump stroke is adjusted by the adjusting screw 2 located on the top of the carburetor just behind the pump lever Best performance is generally achieved when the pump stroke ends at about 3 4 throttle It is important that the nozzle s direction be cor rectly set The nozzle is held in position by the friction of its O ring seal and can be turned quite easily with a pair of long nosed pliers Rotate the nozzle until the fuel stream strikes the needle This ensures an even distribution of fuel between the front and rear cylinders Nozzle adjust ment should be made with the engine stopped and with a minimum of pump strokes to avoid flooding FINE TUNING THE PILOT SYSTEM NOTE Before you apply any of the information in this section be sure that the basic pilot system is correct Be sure that your engine idles smoothly with the mixture screw between one and three turns out from its seated position The pilot and pilot air jets
13. lly the result of a needle jet that is too large The color of the end of the exhausts is a sign of mixture strength Dark grey with some black is normal for today s lead free gasolines If the exhaust color is black chances are you can reduce the size of the needle jet It may be that you might prefer a needle jet that is slightly on the rich side of the correct range A slightly over rich condition lets a Harley accelerate a little better at very low 1 000 1 500 rpm and with very low throttle settings Be aware that you will lose some fuel economy if you choose to do this TUNING THE JET NEEDLE Like the idle circuit and needle jet the needle within its range of operation has a gradually increasing effect on fuel mixture as the throttle is opened From the time about 1 4 throttle that the tapered section of the needle leaves the mouth of the needle jet it has a major effect on the on the amount of fuel entering the engine Between 1 2 and 3 4 throttle the needle s influence is great est and it contols most of the fuel flow All needle and main jet testing should be done wiht the engine near the middle of its rom range Start your acceleration tests at about 50 mph The best needle position will give the strongest acceleration With the engine at operating temperature acceler ate at 1 2 to 3 4 throttle in top gear from 50 mph or so If acceleration seems soft or flat and the engine is slow to respond when the throttle is q
14. outside the 1 4 to 3 1 2 turn range the carburetor s mixture will be either too lean or too rich as the throttle is just being raised off the idle position Once you have a good idle with the screw within this range you may proceed to the next stage tuning the needle jet The pilot air jet is selected while riding at 15 to 30 mph If the engine surges or detonates lean the pilot air jet should be reduced in size Example 1 1 to 1 0 If on the other hand the engine misfires or there is after burn popping from the exhaust rich it would indicate that pilot air jet is too small If the air jet is changed the pilot screw should be re checked and adjusted if needed As mentioned earlier the idle circuit has an impor tant effect on mixtures up to quarter throttle However the idle system s effect on mixture strength overlaps the effect of the neelde jet in this range If the idle circuit is incorrectly adjusted it will not be possible to get the needle jet correct TUNING THE NEEDLE JET It is unlikely that you will need to change the needle jet from the one supplied in your Mikuni HS40 However in case you do you should be aware of how it works and how to tell if the one you have it too large rich or too small for your particular engine set up The needle jet s effect on mixture is limited from about 1 16 throttle where the main system begins to deliver fuel to 1 4 throtlle when the tapered section of the needle begins
15. uickly opened from the 1 2 to the 3 4 throttle position the mixture is too lean Raise the needle one notch and repeat the test One the other hand if acceleration is crisp but the engine hesitates or staggers when the throttle is suddenly shut down from 3 4 to 1 2 throttle the mixture is too rich Lower the needle one notch and repeat the test The needle position will be correct when accelera tion is crisp at mid rpm yet the engine does not load up during throttle shut down TUNING THE MAIN JET You could in fact remove the main jet from your Mikuni HS40 and the engine would run fine until the throttle was near the 3 4 mark The needle and needle jet restrict the amount of fuel getting into the engine until about that position There is no point in changing main jets if a mixture problem exists below the 3 4 throttle position The main jet is the last jet you need to deal with and is the easiest to get right if you have the room The most effective method for getting the main jet right is to measure the time required to accelerate between two points The start and end marker should be spaced so that starting at about 35 mph at the first marker will have you going past the second at near 55 mph Select markers telephone poles do nicely that are far enough apart on a safe road to meet the conditions given above As you pass the first marker roll the throttle fully open Note the rpm or speed at the second marker The jet that g
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