9915083 1999-2000 Vol. III ATV Service Manual Chapter
Contents
1. L 325 425 Under Headlamp Cover 335 500 Above Oil Tank Polaris Sales Inc 4 4 99 FUEL SYSTEM CARBURETION Mikuni J et Part Numbers Pilot ets Part Numbers Main J et Part Numbers 4 Cycle Mikuni VM 2 Cycle Mikuni BST Carburetor JetNo PartNo Jet Number Part Number 125 3130554 A 120 3130887 CE 127 5 3130560 130 3130561 Hex Main J et Part Numbers 192 9 S De Mikuni VM 2 Cycle 1 56 137 5 3130564 es Parno E 142 5 3130566 145 3130567 147 5 3130568 12s as eB 3130871 157 5 3130572 160 3131141 167 5 3131144 170 3131145 Pilot J et Part Numbers 4 Cycle Mikuni BST Carburetor JetNumber Part Number 40 0 3130624 42 5 3130526 4 99 4 5 Polaris Sales Inc FUEL SYSTEM CARBURETION Fuel Tank Assembly Oil Tank Assembly Typical 2 Cycle model with tank mounted fuel valve Details vary by model Refer to parts manual Forward Fuel tank mounted valve Polaris Sales Inc 4 6 4 99 Pilot J et From idling to low speeds the fuel supply is metered by the pilot jet There are several air bleed openings in the sides of the pilot jet which reduce the fuel to mist The number stamped on the jet is an indication of the amount of fuel in cc s which passes through the jet during a one minute inter val under a given set of conditions Pilot Air Screw The pilot air screw controls the fuel mixture from idle to low speeds The tapered tip o2. Jet Needle E Clip Position Refer to Page 4 5 for J et Part Numbers ONOARWNS 9 10 Carburetor Assembly Screw Throttle Valve Cover Diaphragm Jet Block Assembly Spring Diaphragm Assembly Ring E Ring Ring 11 Needle Jet 12 Cover 13 O Ring 14 Throttle Shaft Assembly 15 Ring 16 Seal 17 Spring 18 Packing 19 E Ring 20 Cap 21 Screw 22 Drain Screw 23 O Ring 24 Washer 25 Adjuster 26 Spring 27 Pilot Jet 28 Main Jet 29 Washer 30 Jet Needle 31 Float Assembly 32 Float Body Assembly 33 Float Pin 34 Needle Valve 35 O Ring 36 O Ring 37 Filter 38 Screw 39 Screw 40 Screw 41 Guide Holder 42 Spring 43 Plunger Assembly 44 Spring Washer 45 Screw 46 Air Jet 47 Cable Guide 48 Spring 49 Ring 50 Adjust Screw 51 Screw and Washer Assy Polars Sales Inc 4 22 4 99 FUEL SYSTEM CARBURETION Mikuni BST 40 Carburetor Exploded View Cover Diaphragm Spring Spring Seat E Clip Spacer Jet Needle Diaphragm Assembly Throttle Valve 9 Cable Guide 10 Spring 11 Adjuster Cable 12 Drain Plug 13 O Ring 14 Jet Main 15 Spacer Ring 16 Drain Screw 17 O Ring 18 Float Assembly 19 Plug 20 Pilot Screw 21 Spring 22 Washer 23 O Ring 24 Pilot Jet 25 Valve Inlet Needle 26 Shaft Seal 27 O Ring 28 Air Jet 29 Choke Plunger Guide 30 Spring 31 Choke Plunger 32 Cap 33 O Ring 34 Jet Block Assemb
3. Polaris Sales Inc 4 34 4 99
4. System Fuel Level ed Pipe Needle and Seat specified fuellev All systems Control Floats Float Pins el in float chamber carbure All throttle ranges tor float bowl Venting Vent Passages in Carbure Supplies atmospheric pres All systems tor Vent line sure to fuel in float chamber All throttle ranges Starter Choke Enrichment Choke Lever Cable Choke Supplies additional fuel air All throttle ranges Plunger Return Spring mixture necessary for cold Greatest effect at low Carb Passages Starter Jet starting throttle settings and idle passage in float bowl speeds Pilot Idle System Pilot Jet Passageways Pilot Primarily supplies fuel at idle Mainly idle to 1 4 throttle Air Screw with Spring By and low throttle settings Minimal effect after 1 2 pass Port Beneath Throttle throttle Slide Air Jet Pilot Outlet Throttle Valve Cutaway Main System Main Jet Main Air Passage Supplies fuel at mid range 1 4 to full throttle Needle Jet Jet Needle and high throttle settings Throttle Valve 4 99 4 15 Polars Sales Inc FUEL SYSTEM CARBURETION Vent Systems 2 Cycle Vent Systems 2 Cycle The fuel tank and carburetor float bowl vent lines supply atmospheric pressure to the fuel in the tank and float bowl The lines must be free of kinks and restrictions to prevent lean mixture and possible engine damage Vent lines must be properly routed to prevent damage to the line and to prevent contaminants from ente
5. as the amount of air is increased Throttle Valve Throttle Stop Idle Speed Screw 4 99 4 11 Polaris Sales Inc FUEL SYSTEM CARBURETION Slide Cutaway 1 8 3 8 Throttle 2 Cycle Throttle valve cutaway effect is most noticeable at 1 4 throttle opening The amount of cutaway is pre determined for a given engine to maintain a 14 1 air fuel ratio at part throttle A steep angle would indicate a fairly lean mixture because there is less resistance to air flow A flat angle would provide a much richer mixture because there is more resistance to air flow The venturi shape can be adjusted for each engine s breathing characteristics by using a different valve cutaway angle A number will be stamped into the bottom of the valve e g 2 5 indicating the size of the cutaway The higher the number the steeper the angle Throttle Valve Jet Needle x el s a Air Jet E 3 By pass Pilot Outlet Engine Side Larger Smaller Leaner Richer Polaris Sales Inc 4 12 4 99 FUEL SYSTEM CARBURETION J et Needle Needle J et 3 8 3 4 Throttle 2 Cycle The jet needle and needle jet have the most effect between 3 8 and 3 4 throttle opening Some mixture adjust ment can be accomplished by changing the location of the E clip on the needle Moving the clip down raises the needle in the jet passage and richens the mixture Moving the clip up lowers the
6. duct Air filter dirty plugged Choke plunger sticking incorrectly adjusted choke Choke cable binding or improperly routed Incorrect pilot air fuel screw adjustment Faulty inlet needle and seat Faulty inlet needle seat O Ring Float level too high Poor fuel quality old fuel Loose jets Worn jet needle needle jet or other carburetor parts Dirty carburetor air bleed passages or jets Weak or damaged vacuum piston return spring 4 Strokes Fouled spark plug Poor Idle Symptoms Idle too high S Idle adjusted improperly idle mixture screw damaged S Sticky vacuum slide 4 Strokes or throttle valve 2 strokes S Throttle cable sticking improperly adjusted routed incorrectly S Choke cable sticking improperly adjusted routed incorrectly 4 99 4 33 Polars Sales Inc FUEL SYSTEM CARBURETION Troubleshooting Idle Too Low S Choke cable bending or incorrectly adjusted S Idle speed set incorrectly S Idle mixture screw misadjusted or damaged S Belt dragging S Ignition timing incorrect S Worn jet needle needle jet Erratic Idle S Choke cable bending or incorrectly adjusted n n n n n n n n n n WN Throttle cable incorrectly adjusted Air leaks dirty carburetor passages pilot circuit Pilot mixture screw damaged or adjusted incorrectly Tight valves Ignition timing incorrect Belt dragging Dirty air cleaner Engine worn Spark plug fouled Idle speed set incorrectly speed limiter Worn jet needle needle jet
7. pass Pilot Outlet Pilot Jet Needle Jet Main Jet Polaris Sales Inc 4 14 4 99 FUEL SYSTEM CARBURETION Component Effect vs Throttle Opening 2 Cycle Mikuni Fuel Delivery 2 Cycle The throttle opening chart below demonstrates component relationship to fuel flow versus throttle valve opening The pilot system s main function is that of a low speed jet Its most effective range of fuel delivery is from idle to approximately 3 8 throttle valve opening The throttle valve controls the rate of engine air by its movement up and down in the carburetor venturi At small throttle openings the air flow is regulated chiefly by the valve cutaway with greatest effectiveness at 1 4 throttle opening Throttle valves are numbered 1 0 1 5 2 0 etc according to the size of the cutaway Decreasing the cutaway number will increase the amount of fuel delivered in its effective range The jet needle and needle jet have an effective operating range from approximately 1 8 to 7 8 throttle opening The amount of fuel delivered during this range relies upon the jet needle clip position as well as the needle jet size and other specifications The main jet affects fuel delivery at 1 4 throttle and consistently increases to full throttle opening INCREASING DECREASINGT THROTTLE OPENING 2 Cycle Throttle Valve Cut Away Jet Needle Needle Jet Carburetor L Function 2 Cycle System Main Main Components NN a
8. 100 3000 6000 145 135 Pilot Screw 2 1800 2700 Pilot Air Jet 160 60009009 Valve Seat 1 5 8000 12000 Fuel Octane R M 2 87 Non Oxygenated or 89 Oxygenated Pilot screw in 1 2 turn Polaris Sales Inc 4 1 4 99 FUEL SYSTEM CARBURETION Fuel Tank Assembly Oil Tank Assembly Typical 2 Cycle model with tank mounted fuel valve Details vary by model Refer to parts manual Forward Fuel tank mounted valve l d E Mut Polaris Sales Inc 4 4 99 FUEL SYSTEM CARBURETION Fuel Tank Assembly S Vent line routed Typical 2 Cycle or 4 Cycle model smoothly into with frame mounted fuel valve l center of steering Details vary by model i post Forward Outlet Silver fitting short stand pipe to Reserve marked 2 on fuel valve Gold fitting tall stand pipe to ON marked 1 on fuel valve Tank to fuel valve connection Gold fitting tall stand pipe to ON marked 1 on fuel valve Reserve Silver fitting short stand pipe to Res marked 2 on fuel valve 4 99 4 3 Polaris Sales Inc FUEL SYSTEM CARBURETION Fuel Flow Diagrams Fuel System Diagram 2 Stroke Models Fitting Screens Fuel Filter Fuel Valve Carburetor Vent Fuel System Diagram 4 Stroke Models Fuel Tank Vent Fuel Tank Fitting Screens L Fuel Pump Carburetor Vent Fuel Valve
9. 4 throttle Mixture Screw with Spring and low throttle settings Minimal effect after 1 2 Washer and Sealing O Ring throttle Bypass Ports Behind Throttle Plate Pilot Air Jet Pilot Outlet Throttle Plate Main System Main Jet Main Air Jet Main Supplies fuel at mid range 1 4 to full throttle Air Passage Needle Jet Jet and high throttle settings Needle Vacuum Slide Throttle Plate Vent Systems 4 Cycle CV Carburetor The carburetor float bowl vent lines supply atmospheric pressure to the fuel in the float bowl The lines must be To frame free of kinks and restrictions and be properly routed to al To frame low fuel to flow in the proper amount and to prevent con _ lt taminants from entering the carburetor P R M c L I Polaris Sales Inc 4 18 4 99 FUEL SYSTEM CARBURETION Mikuni CV Carb Operation The constant velocity carburetor used on Polaris 4 Cycle ATVs and 6x6 incorporates a mechanically operated throttle plate and a vacuum controlled slide valve vacuum slide The venturi cross sectional area in the carbure tor bore is increased or decreased automatically by the vacuum slide which moves according to the amount of negative pressure less than atmospheric present in the venturi A diaphragm attached to the top of the vacuum slide is sealed to the slide and to the carburetor body forming two chambers The chamber above the diaphragm is connected to the venturi area by a drilled orifice in the cente
10. CHAPTER 4 FUEL SYSTEM CARBURETION Jetting Guidelines 4 1 Main Jet Selection 4 1 Fuel and Oil Tank Asm Exploded View 2 Cycle 4 2 Fuel Tank Asm Exploded View 4 Cycle 4 3 Fuel Flow Diagrams 4 4 Main Jet Pilot Jet Part Numbers 4 5 2 Cycle Section Carburetor Component Function 2 Cycle 4 6 4 9 Carburetor Circuit Operation 2 Cycle 4 10 4 15 Carburetor Vent Systems 2 Cycle 4 16 Float Height Adjustment 2 Cycle 4 16 Needle and Seat Leakage Test 2 Cycle 4 16 Exploded View Mikuni VM 30 34 SS Carburetor 4 17 4 Cycle Section CV Carburetor System Function 4 Cycle 4 18 CV Carburetor Vent System 4 Cycle 4 18 CV Carburetor Operation 4 19 4 21 Exploded View Mikuni BST 34 Carburetor 4 22 Exploded View Mikuni BST 40 Carburetor 4 23 Disassembly Notes CV Carburetor 4 24 4 25 Cleaning CV Carburetor 4 26 Inspection CV Carburetor 4 27 Assembly CV Carburetor 4 28 Adjustment CV Carburetor 4 29 4 30 Fuel Pump Fuel Pump Disassembly Inspection Assembly 4 31 Fuel Pump Exploded View 4 32 Troubleshooting 4 33 4 34 FUEL SYSTEM CARBURETI
11. ON Jetting Guidelines Jetting Guidelines Changes in altitude and temperature affect air density which is essentially the amount of oxygen available for combustion In low elevations and cold temperatures the air has more oxygen In higher elevations and higher temperatures the air is less dense Carburetors on Polaris ATV and 6x6 vehicles are calibrated for an altitude of 0 3000 ft 0 900 meters and ambi ent temperatures between 40 and 80 F 5 to 26 C Carburetors must be re calibrated if operated outside the production temperature and or altitude range The main jet installed in production is not correct for all altitudes and or temperatures In addition air screw pilot screw adjustments may be required to suit operating conditions CAUTION A main jet that is too small will cause a lean operating condition and may cause serious engine damage Select the correct main jet carefully for elevation and temperature according to the charts in the General Specifications Chapter or in the Owner s Safety and Maintenance Manual for each particular model Air Screw 2 Cycle Pilot Screw 4 Cycle Adjustment NOTE Maximum engine efficiency and horsepower are directly related to proper carburetor and clutch settings The jetting charts should be used as a guideline for selecting optimum jetting for varying temperature and altitude conditions Air screw or fuel screw adjustment will affect mixture from approximately idle t
12. Q F OQ N DS 10 Screw Air Adjust Spring Throttle Stop Screw Screw Throttle Stop Pilot Jet Washer Main Jet Screw and Washer Assy Main Jet O Ring Drain Plug Float Bowl Float Cap Gasket Valve Inlet Needle Arm Float Pin Float Gasket Inlet Valve Carb Body Assy 1 30 Needle Jet E Clip Cap Spring Plate Cable Retainer Jet Needle Washer Valve Throttle Cap Plunger Spring Starter Plunger Spring Air Adj Screw Check Valves To Carburetor 3 4 99 4 17 Polaris Sales Inc FUEL SYSTEM CARBURETION Mikuni CV Carburetor System Components 4 Cycle 4 Cycle CV Carburetor System Function Carburetor Component Function 4 Cycle Float System Inlet Pipe Needle and Seat Maintains specified fuel level All systems Fuel Level Control Float Float Pin in float chamber carburetor All throttle ranges float bowl Venting Vent Passages in Carburetor Supplies atmospheric pres All systems Vent lines 2 into 1 to frame sure to fuel in float chamber All throttle ranges Starter Choke Lever Cable Choke Supplies additional fuel air All throttle ranges Choke Enrichment Plunger Return Spring Carb mixture necessary for cold Greatest effect at low throttle Passages Starter Jet Starter starting settings and idle Bleed Pipe Pilot Idle System Pilot Jet Passageways Pilot Primarily supplies fuel at idle Mainly idle to 1
13. e middle of the needle to the lower end The top is fixed to the center J et Needle of the throttle valve by the needle clip and the tapered end 1 Leaner extends into the needle jet Fuel flows through the space 3 between the needle jet and jet needle This space does not 4 Groove vary until the throttle reaches the 1 4 open point At that 5 time the tapered portion of the needle begins to move out of the jet affecting fuel flow as the opening enlarges If the needle clip is changed from the standard position to a lower groove the needle taper starts coming out of the jet sooner resulting in a richer mixture Moving the clip higher pro duces a leaner mixture If the taper is worn due to vibration fuel flow may be significantly affected Needle J et The needle jet works in conjunction with the jet needle to regulate fuel flow rate An air bleed opening in the side of the needle jet brings in air measured by the air jet This air initiates the mixing and atomizing process inside the needle jet Mixing is augmented by a projection at the needle jet outlet called the primary choke The letter number code stamped on the jet indicates jet inside diameter Opening faces engine TA Washer y Main Jet Throttle Opening vs Fuel Flow In a full throttle condition the cross sectioned area be Medium Large tween the jet needle and the needle jet is larger than Clearance Clearance the cross sectioned area of the main jet The main jet
14. f the air screw projects into the air passage leading to the pilot jet air bleeds By turning the screw in or out the cross sectional area of the air passage is varied in turn varying the pilot jet air supply and changing the mixture ratio Air Fuel Mixture Ratio A carburetor with a piston type throttle valve is also called a variable venturi type carburetor In this type of carburetor the needle jet and jet needle serve to control a proper air fuel mixture ratio at the medium throttle valve opening be tween 1 4 and 3 4 opening Having the proper needle jet and jet needle has a major impact on engine performance at partial load The jet needle tapers off at one end and the clearance be tween the jet needle and the needle jet increases as the throttle valve opening gets wider The air fuel mixture ratio is controlled by the height of the E ring inserted into one of the five slots provided in the head of the jet needle The chart at right shows the variation of fuel flow based on the height of the E ring Fuel flow FUEL SYSTEM CARBURETION Component Functions 2 Cycle Pilot Jet Indicator Number E Clip Position Vs Fuel Flow 15 50 75 100 Throttle Valve Opening 4 99 4 7 Polaris Sales Inc FUEL SYSTEM CARBURETION ComponentFunctions 2 Cycle Jet Needle The jet needle has five adjustment grooves cut into the up per portion and is tapered from approximately th
15. her Assembly Screw and Washer Assembly Pressure Regulator Fuel Inlet Fuel Outlet 0o O Q gt QO N Polaris Sales Inc 4 32 4 99 FUEL SYSTEM CARBURETION Troubleshooting Fuel Starvation Lean Mixture Symptoms Hard start or no start bog backfire popping through intake exhaust hesitation detonation low power spark plug erosion engine runs hot surging high idle idle speed erratic S NNnNnNNnNNnNnNNNHNHNNHNHNVHnWNNnNWN WH n No fuel in tank Restricted tank vent or routed improperly Fuel lines or fuel valve restricted Fuel filter plugged Carburetor vent line s restricted Plugged or restricted inlet needle and seat screen or inlet passage Clogged jets or passages Float stuck holding inlet needle closed or inlet needle stuck Float level too low Fuel pump inoperative 4 Strokes Air leak at impulse line 4 Strokes Restricted impulse line kinked pinched 4 Strokes Intake air leak throttle shaft intake ducts airbox or air cleaner cover Ruptured vacuum slide diaphragm Vacuum slide stuck closed or sticky 4 Strokes Improper spring 4 Strokes Jet needle position incorrect Incorrect pilot screw adjustment Rich Mixture Symptoms Fouls spark plugs black sooty exhaust smoke rough idle poor fuel economy engine runs rough misses poor performance bog engine loads up backfire S n n n n n n n n Q G n WN n Air intake restricted inspect intake
16. le and needle Inspect this jet to prevent a rich condition 2 Inspect the inlet needle tapered surface for any sign of wear or damage Be sure the spring loaded pin is free moving and returns freely when pushed The inlet needle and seat should be pressure tested after Ms Worn Deposits assembly Good Condition Dep 4 99 4 27 Polaris Sales Inc FUEL SYSTEM CARBURETION Carburetor Assembly Mikuni CV Carburetor Assembly 1 4 Inspect the diaphragm A for holes deterioration or damage Make sure the diaphragm is pliable but not swollen The diaphragm should fit properly in the carburetor body Replace diaphragm assembly if diaphragm is damaged Replace parts in proper order The spring seat washer B is stepped and must be placed on TOP of E Clip C Spacer washer D must be installed below the E Clip Refer to parts manual for more information Be sure the tab E on outer edge of diaphragm is positioned properly in the carburetor body Install the pilot mixture screw spring washer and O ring as an assembly Lubricate the O Ring with oil or light grease before installation CAUTION Do not damage the O ring during installation Turn the screw in until it lightly contacts the seat Back out the specified number of turns NOTE The final pilot idle mixture must be adjusted with the engine running Refer to Page 2 12a Pilot Mixture Screw Base Setting Turns Out Refer to General Specifica
17. ly 35 Needle Jet 36 Case ONDAN Refer to Page 4 5 for J et Part Numbers 4 99 4 23 Polaris Sales Inc FUEL SYSTEM CARBURETION Carburetor Disassembly Notes Mikuni CV Use the following disassembly assembly and inspection techniques to service a CV carburetor 1 Remove carburetor diaphragm chamber cover with a ratchet style screwdriver DO NOT use an impact driver to remove the screws or carburetor may be permanently damaged 2 Use a small spring loaded center punch to remove pressed float pin 3 Remove inlet needle seat retaining screw along with plate and carefully remove needle seat NOTE Do not use a pliers to remove the seat or permanent damage may occur Polaris Sales Inc 4 24 4 99 FUEL SYSTEM CARBURETION Carburetor Disassembly Mikuni CV 4 Do not misplace the pilot mixture screw spring flat washer or O Ring If anti tamper plug is installed in pilot screw cavity refer to Maintenance chapter 2 for removal procedure Washer Spring 5 NOTE The starter jet is not removeable Starter Jet 4 99 4 25 Polars Sales Inc FUEL SYSTEM CARBURETION Carburetor Cleaning Mikuni CV Carburetor Cleaning 1 Thoroughly clean the carburetor body jets and all passages with carburetor cleaner or electrical contact cleaner Protect eyes from contact with cleaner If you get cleaner in your eyes or if you swallow cleaner see your doctor immediately Some carbure
18. needle in the jet passage and leans the mixture Letter and number codes are stamped into the needle and the jet indicating sizes and tapers of each Clip Position Shown in 3 Throttle Valve J et Needle Air Jet By pass Pilot Outlet Pilot Jet Needle J et Main Jet Leaner Richer Jet Needle E Clip Position 4 99 4 13 Polaris Sales Inc FUEL SYSTEM CARBURETION Main System 3 4 to Full Throttle 2 Cycle The main system is designed for delivering fuel between low speed and high speed operation This system is made up of the jet needle needle jet and main jet The main system begins to take effect as soon as there is enough air flow into the carburetor venturi to draw fuel up through the main jet and needle jet assembly This system works in conjunction with the needle jet system During low speed driving there is very little clearance between the jet needle and the needle jet therefore very little fuel from the main jet can pass between the jet needle and the needle jet As the throttle valve opening is increased the tapered jet needle is raised farther out of the needle jet allowing greater fuel flow Under full throttle opening the cross sectioned area of clearance between the jet needle and the needle jet becomes greater than the cross sectioned area of the main jet Thus the main jet is now controlling the amount of fuel flow Throttle Valve Jet Needle Air Jet By
19. needle seat 4 and around the inlet needle 2 As the fuel fills the float cham ber the float 1 rises and forces the inlet needle against the seat shutting off the orifice in the seat When fuel level is up in float chamber floats are up and needle valve remains pushed up against valve seat Under this condition no fuel enters the float chamber As the fuel level falls floats go down and needle valve unseats itself to admit fuel into the chamber In this manner the needle valve admits and shuts off fuel alternately to maintain a practically constant fuel level inside the float chamber Main System As throttle valve 1 is opened engine speed rises and this increases negative pressure in the venturi Consequently the vacuum slide 2 moves upward The fuel in float chamber 3 is metered by main jet 4 and the metered fuel enters needle jet 5 in which it mixes with the air admitted through main air jet 6 to form an emul sion The emulsified fuel then passes through the clearance between needle jet 5 and jet needle 7 and is discharged into the venturi A Mixture proportioning is accomplished in needle jet 5 the clear ance through which the emulsified fuel must flow is determined ultimately by throttle position and vacuum slide height FUEL SYSTEM CARBURETION Mikuni CV Carb Operation 4 99 4 21 Polaris Sales Inc FUEL SYSTEM CARBURETION Mikuni BST 34 Carburetor Exploded View
20. o 1 4 throttle setting Refer to Maintenance Chapter 2 for complete adjustment procedure and the following guidelines for minor altitude adjust ments Air Screw 2 stroke models Turn the air screw in clockwise 1 4 turn for each 30 below 60 F Turn the air screw out counterclockwise 1 4 turn for each 30 above 60 F Fuel Screw 4 stroke models Turn the fuel screw in clockwise 1 4 turn for each 30 above 60 F Turn the fuel screw out counterclockwise 1 4 turn for each 30 below 60 F Main Jet Selection IMPORTANT The following guidelines must be followed when establishing a main jet setting 1 Select the lowest anticipated temperature at which the machine will be operated 2 Determine the lowest approximate altitude at which the machine will be operated 3 Select the correct main jet from the chart 4 Clutching changes may also be required for changes in elevation Refer to clutching charts in General Specifications Chapter 1 for recommendations MODEL 500 6X6 EXAMPLE ONLY Refer to Chapter 1 MODEL NUMBER A99AESOAA Production Main Jet outlined in BOLD ENGINE MODEL EH50PLE06 JETTING CHART CARBURETION AMBIENT TEMPERATURE Type a fa aa aa ae oa a ae BST 34 Mikuni Below 0 F 0 t0 40 F 40 to 80 F Main Jet C 140 Below 18 C 18 to 5 C 5 to 26 C Pilot Jet 40 426 C Needle Jets G68 E Throttle Valve
21. ormed on some models if the drain hose fitting is accessible Be sure to re attach the bowl drain hose after performing the test A fuel level test allows you to observe the height of the fuel in the float bowl without removing the carburetor The fuel level can be ob served with the engine either running or shut off however engine must run briefly to allow fuel level to stabilize Be sure to review all fuel warnings on page 4 41 and 4 44 1 Attachaclear line to drain fitting Be sure line fits tightly on fitting Position hose along side of carburetor as shown 2 Open bowl drain screw by turning counterclockwise approximately two turns Start and run engine for 3 to 5 seconds to allow fuel level to stabilize in the line If level is out of specification remove carburetor and inspect inlet needle and seat float height passages etc Fuel Level Bowl Mating Surface Polars Sales Inc 4 30 4 99 FUEL SYSTEM CARBURETION Fuel Pump Fuel Pump 4 Cycle models are equipped with a pressure regulated fuel pump about 1 3 PSI The pump is located under the headlight cover at the front of the machine or on lower left side of fuel tank near oiltank Refer to illustration on following page for fuel pump component identification To test the fuel pump 1 Turn fuel off 2 Disconnect impulse line from pump 3 Connect Mity Vac PN 2870975 to the impulse line fitting on the pump 4 Apply 5 inches Hg vacuum to the p
22. r of the vacuum slide The chamber below the diaphragm is vented to atmospheric pressure by a passage on the air box side of the carburetor A spring installed in the center of the vacuum slide dampens the slide movement and assists the return of the slide When the throttle plate is opened and engine speed begins to increase the pressure in the venturi and therefore in the chamber above the diaphragm becomes significantly lower than atmospheric Atmospheric pressure in the chamber below the diaphragm forces the diaphragm upward raising the slide against spring pressure When the pressure above and below the diaphragm are nearly equal the slide moves downward under spring pressure Raising or lowering the slide increases or decreases the cross sectional area in the venturi and therefore the air velocity in the venturi is kept relatively constant This provides improved fuel atomization and optimum fuel air ratio Low Pressure Diaphragm Low Pressure Diaphragm From Venturi From Venturi Air Box Vacuum x 0772777777777 2 y ZZ 7777727 LEES un Pressure C A H er A Slide AT uper 1 A k A IT H H p 4 d d 4 H Z a 1 4 H gt gt gt EN SG S SSS amd SY S OSSSSAMISASSSS De N IW Z PAT NE S j ta ANS m Venturi Low Pressure um Ai Flow nm Ai Flow lt Low Pressure lt lt Note Diagrams are for explanation of theory only and are not tr
23. r for starting the engine are metered with entirely independent jets The fuel metered in the starter jet is mixed with air and is broken into tiny particles in the emulsion tube The mixture then flows into the plunger area mixes again with air coming from the air intake port for starting and is delivered to the engine through the fuel discharge nozzle in the optimum air fuel ratio The starter is opened and closed by means of the starter plunger The starter type carburetor is constructed to utilize the negative pressure of the inlet pipe so it is important that the throttle valve is closed when starting the engine Cable Adjuster Lock Nut Plunger Spring lt Plunger Cap Throttle Valve Starter Plunger Inlet Starter Air Poe td r Starter Jet Polaris Sales Inc 4 10 4 99 FUEL SYSTEM CARBURETION Pilot System 0 3 8 Throttle 2 Cycle The pilot system s main function is to meter fuel at idle and low speed driving Though its main function is to supply fuel at low speed it does feed fuel continuously throughout the entire operating range Fuel for the pilot jet is drawn from the float bowl mixed with air regulated by the air screw and delivered to the engine through the pilot outlet The mixture is regulated to some degree by adjusting the air screw When the air screw is closed the fuel mixture is made richer as the amount of air is reduced When the air screw is opened the mixture is made more lean
24. ring the carbu retor or fuel tank A one way check valve is in place on the bowl overflow hose to allow overflow fuel out and prevent water or other contaminants from entering the bowl See photo below right To T n Check Valve Float Height 2 Cycle 1 Invert the carburetor and remove float bowl 2 Restthe float tongue lightly on the inlet needle valve pin without compressing the spring 2 Parallel 3 Measure height from float bowl mating surface to float arm as shown Both sides of float arm must be parallel to each other Use float adjustment tool PN 2872314 or a vernier caliper When measuring height be sure inlet needle valve spring is not compressed If adjustment is necessary bend the tongue slightly Float Height VM 30 Parallel to Gasket Surface Imm V aralle aSKe ulface NeedIdma d Seat Leakage Test 2 Cycle 1 Install the float bowl Invert the carburetor and install a Mity Vac PN 2870975 to the fuel inlet fitting Apply 5 PSI pressure to inlet fitting The needle and seat should hold pressure indefinitely If not inspect needle and seat and seat O ring or gasket Polaris Sales Inc 4 16 4 99 2 Cycle Mikuni VM30SS VM34SS Carburetor Exploded View E Refer to Page 4 8 for Jet Part Numbers Jet Needle E Clip Position 1 rone Use a spring loaded center punch to remove press fit float pin FUEL SYSTEM CARBURETION O
25. therefore has greater control over fuel flow Closed One half Full Throttle Throttle Throttle Polaris Sales Inc 4 8 4 99 Throttle Valve The throttle valve controls the rate of engine air intake by moving up and down inside the main bore At small throttle openings air flow control is performed chiefly by the cutaway By controlling air flow the negative pressure over the needle valve is regulated in turn varying the fuel flow The throttle valves are numbered 1 0 1 5 2 0 etc according to the size of the cutaway The higher the number the leaner the gasoline air mixture Main J et When the throttle opening becomes greater and the area between the needle jet and jet needle increases fuel flow is metered by the main jet The number on the jet indicates the amount of fuel CCs which will pass through it in one minute under controlled conditions Larger numbers give a greater flow resulting in a richer mixture Main jets are screwed directly into the needle jet base FUEL SYSTEM CARBURETION Component Functions 2 Cycle Throttle Valve Engine Side Air Box Side Cutaway a 3 Higher cutaway number leaner mixture Main J et i Mikuni Hex Style Main Jet Larger Number Richer Mixture 4 99 4 9 Polaris Sales Inc FUEL SYSTEM CARBURETION Starter System Closed Throttle 2 Cycle Mikuni carburetors use a starter system rather than a choke In this type of carburetor fuel and ai
26. tions Chapter 1 Pilot Scre W Polaris Sales Inc 4 28 4 99 FUEL SYSTEM CARBURETION Carburetor Adjustment Mikuni CV Float Height Adjustment 1 Place the carburetor on a level surface as shown at right to remove weight from float arm In this position the float tongue will rest lightly on the inlet needle valve pin without compressing the spring 2 Measure the height from the float bowl mating surface to the top of step in float as shown Both sides of float should be parallel to each other The measurement should be made at the mid point on the top of the float using float adjustment tool PN 2872314 or a vernier caliper When measuring the height be sure the inlet needle valve spring is not compressed Float Height Std BST 34 13 0mm 51 1 mm BST 40 14 7mm 58 1 mm 3 If adjustment is necessary bend the tongue slightly Float arms even Be sure float arms are even on left and right side ooi aie Sven Bend to adjust float 4 99 4 29 Polaris Sales Inc FUEL SYSTEM CARBURETION Carburetor Adjustment Mikuni CV Needle and Seat Leakage Test 1 Install the float bowl Invert the carburetor and install a Mity Vac PN 2870975 to the fuel inlet fitting Apply 5 PSI pressure to inlet fitting The needle and seat should hold pressure indefinitely If not inspect needle and seat and seat O ring Mity Vac PN 2870975 Fuel Level A fuel level test can be perf
27. tor cleaners are extremely caustic and extended periods of soaking can loosen the adhesive sealer on the passage drill way plugs Do not soak rubber or plastic components such as the vacuum slide diaphragm needle seat screen or O Rings in caustic cleaning solutions Irreparable damage may occur Do not use agitator type carburetor cleaning equipment Rubber parts must be cleaned with mild detergent and hot water only 2 Ifthe carburetor is extremely dirty or contaminated with fuel residue and varnish soak for short periods only in carburetor cleaner and rinse in hot water 3 Replace the jets if they are extremely dirty or have a buildup of fuel residue or bacterial growth Even a small amount of residue will reduce the flow characteristics of the jet 4 Verify all passages and jets are unobstructed by spraying electrical contact cleaner through the passages CAUTION Do not use wire or welding tip cleaners on the jets as the orifice size may be altered 5 Use low pressure air to dry carburetor body and all components Polaris Sales Inc 4 26 4 99 FUEL SYSTEM CARBURETION Carburetor Inspection Mikuni CV Carburetor Inspection 1 Inspect jet needle and needle jet for wear Look for discoloration shiny spots or and area that looks different than the rest of the needle The middle to upper portion of the needle where it contacts the needle jet is the most likely wear point If jet needle shows signs of wear replace both need
28. ue representations of Mikuni BST 34 BST 40 carburetor 4 99 4 19 Polaris Sales Inc FUEL SYSTEM CARBURETION Mikuni CV Carb Operation Pilot Idle and Slow System This system supplies fuel during engine operation with throttle valve closed 1 or slightly opened The fuel from float cham ber 2 is metered by pilot jet 3 where it mixes with air coming in through pilot air jet 4 The mixture then goes up through pilot passage to pilot screw 5 A part of the mixture is discharged into the main bore out of bypass ports 6 The remain der is then metered by pilot screw and dis charged into the main bore through pilot outlet 7 Starter System Choke or Enrichment When the choke cable 1 is activated the starter plunger 5 is lifted off the seat Fuel is drawn into the starter circuit from the float chamber 2 through the starter jet 3 Starter jet meters this fuel which then flows into starter pipe 4 and mixes with the air 7 coming from the float chamber The mixture rich in fuel con tent reaches starter plunger and mixes again with the air coming through a pas sage 8 extending from underneath the diaphragm The rich fuel air mixture for starting is discharged through starter out let 6 in the the main bore Polaris Sales Inc 4 20 4 99 Float System Fuel enters the float chamber 3 by means of the inlet pipe and passage through a screen on the back of the inlet
29. ump fitting The diaphragm should hold vacuum indefinitely If fuel is present in the impulse line or vacuum chamber of the pump the diaphragm is ruptured and the pump diaphragms must be replaced Fuel Pump Disassembly 1 Refer to illustration on following 2 Remove the screws from the pump diaphragm cover Note the location of the two longer screws 3 Remove the diaphragm cover gasket diaphragm and valve body gasket 4 Remove the outlet check valve cover diaphragm and gasket Fuel Pump Inspection Assembly 1 Inspect inlet and outlet check valves for cracks warpage or damage Inspect the diaphragms for cracks holes or swelling 2 To clean the valves or pump body remove the set screw and washer Remove the valve and wash with soap and water Carburetor cleaner may be used to clean the pump body when the check valves are removed CAUTION Some carburetor cleaners are very caustic and should not be used to clean the non metal parts of the fuel pump 3 Check the sealing surfaces of the pump body and covers Carefully remove all traces of old gasket and check the surfaces for damage Replace diaphragms and gaskets as a set 4 Reassemble the pump in the reverse order of disassembly Tighten all screws evenly 4 99 4 31 Polaris Sales Inc FUEL SYSTEM CARBURETION Fuel Pump Fuel Pump Exploded View Fuel Pump Assembly Diaphragm Gasket Set Screw and Washer Assembly Screw and Was
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