Transcription

MIKUNI VM CARBURETORSUPER TUNING .UAlnil: MIKUNI !:'M K ! N 2 E RATIONCALIFORNIA 91324 U.S.A.

EXPLODED VIEW: i31 341. Rubber Cap, Throttle Cable2. Cable Adjuster3. Locknut,. Cable Adjuster4. Top, M:l.xing Chamb er5. Tube, Air Vent6. Spring, Piston Valve7. Plate, Needle Retainer8. "E" Ring9. Jet Needle10. Piston Valve11. Needle Jet12. Rubber Cap, Starting System13. Cap, Starter Plunger14. Lever Assy, Starting System15. Leaf Spring, Lever Positioning16. Spring, Starter Plunger17. Plunger, Starting System18. Body, Mixing Chamber19. Gasket, Float Chamber20. Baffle Plate, Flost Chamber21- Pilot Jet22. Pin, Float Arm Hinge23. Float Arm24. Float25. Float Chamber26. Washer, Float Chamber Plug27. Plug, Float Chamber28. Air Jet29. Air Screw30. Spring, Air Adjusting Screw31. Spring, Idle Adjusting Screw32. Screw, Idle Adjusting33. Washer, Needle & Seat Assy34. Needle & Seat Assy35. Cup, Fuel Retaining36. Main Jet37. Plate, Vent Tube Retaining38. Screw, Float Chamber30 29181516617281921273852035936342112237I22738PARTS I /14M21/14K21/141 M20}455VM20/455 H20/4SSVK2.D/455VH20/4SSVK2D/liS531.H20111.20/111 20111.20111.20111.20111.20/111 00041224002/171VM22/1710041224002/171VH36/581' IIZ-0516 12-0516OW2-D516CWZ-D516CIIZ-051619.0081129

ForewordThis manual is intended as a guide for users of Mikuni carburetors who want to learn the adjustingmethod to the best performance from our products. In motorcycles, special tuning of the engine is nowconsidered a routine practice.The arrows that appear in the drawings in this text show the directionin which air, fuel and an air-fuel mixture flows, respectively.Fuel JAir"MixtureMounting angle fore and aft inclination of the carb should not exceed approx 20 degreesfrom horizontal.r- -- - --- -- -- -- - --- ---- ------ --- -- - - -- - - - - -----""]r -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -,IIIIIII !:UIIIIIIIII:IIII:Jet needleI:Throttle valve:II !Air jetFloatNeedlebalve Pilot jetIII-,IIIII : : : - - c:::::; Ai r Fuel"'Mixture'''--Ring

TABLE OF CONTENTS1.What is a carburetor? . . . . . . . . . 12.Air-fuel mixture ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13.Function and construction. . . . 13-1. Low-speed fuel system (pilot system) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-33-2. Main fuel system (main system) . . . . . 33-2-1. Primary type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43-2-2. Bleed type. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43-3. Float system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55 63-4. Starting device (starter system)4;Tuning up of engine and selection of aperture of carburetor . . . . . . . . . . . . . . . . . . . . . 64-1. Selection of aperture of carburetor5. . .:. 6 - 74-2. Sizes of Mikuni carburetors7Carburetor setting75-1. Selection of main jet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .85-2. Selection of needle jet and jet needle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 105-3. Pilot jet and the slow system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10 -115-4. The cutaway size of throttle valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116.Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 27.Tuning up of carburetor for racing. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137-1. The amount of incoming air in relation to meteorological conditions . . . 13

1.Function of a carburetorThe function of a carburetor is to produce combustible air-fuel mixture, by breaking fuel into tinyparticles (in the form of vapor) and by mixing the fuel with air in a proper ratio, and to deliver themixture to the engine. A proper ratio (mixture ratio or air-fuel ratio) means an ideal air-fuel mixture thatcan burn without leaving an excess of fuel or air, Whether the proper mixture ratio is maintained or notholds the key to the efficient engine operation,2.Air-fuel mixture required by the engine (Fig. 1)The ratio of a mixture of fuel and air is called the mixture ratio or the air-fuel ratio and is generallyexpressed by the weight proportion. Theoretically, the amount of air required for complete combustionof 1 gram of fuel under normal conditions is:. t ure rati10". th eoret'rcaI mixgramsofoffuelairMI'xt ure ratio 151 gramIn reality, varying mixture ratios are required for the engine, depending on operating conditions.Although the required mixture ratio varies more or less with the type, the cooling efficiency, etc. of theengine, the mixture ratio shown in Fig. 1 isrequired for ordinary engines. In the high speedrange, the ratio of about 12 to 13 grams of air forAir-fuel mixture10:11 gram of fuel produces the maximum output.12:1However, in the case of an engine with low14:1cooling efficiency, a somewhat richer mixture (10to 12 grams of air against 1 gram of fuel) may beMaximum power mixture ratio.16:1Theoretical mixture ratio18:1required to prevent seizure of the engine.20406080Throttle valve opening (%)100Fig. 13.Functions and constructionThe engine of a vehicle is operated under a wide range of conditions, from idling with the throttlevalve (1) remaining almost closed to the full load (the maximum output) with the throttle valve fullyopened. In order to meet the requirements for the proper mixture ratio under these varying conditions, alow-speed fuel system (the pilot system) and a main fuel system (the main system) are provided inMikuni VM-type carburetors.-1-

3-1.Low-speed fuel system (the pilot system) . Fig. 2 and Fig. 3Since the engine is operated with the throttle valve almost closed at idling or in the low speed range,the velocity of air flowing through the needle jet (2) is slow. Consequently, a negativep essurestrongenough to draw fuel from the needle jet in the main fuel system is not created. The fuel supply duringthis low speed operation is controlled by means of the pilot outlet (3) and the bypass (4) that areprovided close to the engine.When the degree of the throttle valve opening is small at idling, fuelmetered with the pilot jet (5) is mixed with air adjusted in a proper amount by the air screw (6) and isbroken into fine particles.Fig. 3Fig. 2The mixture is again mixed with air coming from the bypass and is drawn into the pilot outlet to mixwith air flowing through the main bore (7). The fuel mixed with air in the optimum ratio at this stagethen goes into the engine. When the throttle valve opening gets wider for low speed operation, the pilotoutlet alone cannot supply the required fuel and the shortage has to be made up with fuel injected fromthe bypass. The adjustment of the mixture ratio during this stage is made by the pilot jet and the airscrew, in the case of a two-hole type fuel system. There is also a one-hole type low speed fuel system-2-

mainly used for carburetors having asmall main bore. The process of producing the air-fuel mixture and ofadjusting the mixture ratio are thesame as in a two-hole type low speedsystem.3-2. On Mikuni VM-type carburetors,the pilot system and the main systemFig. 4are of independent construction.The fuel flow in these two systems isshown in Fig. 5. There are two types ofthe main fuel system; one is a primaryTotal amount of fuel flow""type used widely for 2-cycle enginesand the other is a bleed type whichmay be used for4-cycle engines aswell as for rotary valve type 2-cyclePilot fuel system/engines.Throttle valve opening (%)--Fig. 5-3-

3-2-1.Primary typeWhen the throttle valve is opened about 1/4 or more, the velocity of air flowing through the needlejet increases and also the negative pressure increases to the point where fuel can be sucked in. When theopening of throttle valve is between about a quarter and three quarters, fuel passes through the main jetand, after being metered in the clearance between theneedle jet and the jet needle, it is mixed with air that ismetered in the air jet and atomization of the fuel isaccelerated.The mixture is then injected, after mixing with air flowingthrough the main bore, to the engine in the optimumair-fuel ratio.During this process of operation, thecutaway of the throttle valve serves to control the negativepressure on the needle jet, thereby regulating the amountof fuel that is injected to the engine. When the throttlevalve is opened more than three quarters for high speedoperation, fuel is chiefly metered with the main jet.3-2-2.Fig. 6Bleed type (Fig. 7)The construction of the bleed-type main fuel systemis the same as that of the primary type, except for thebleed hole that is provided in the needle jet (Fig. 7).In the case of the primary type, air that comes from the airjet is mixed with fuel that is metered with the needle jetand the jet needle. The bleed type, on the other hand, isdesigned to hold air in the body section of the needle jetand then to have the air and fuel metered.Fig. 7-4-

3-3.Float system (Fig. 8)The float system serves to maintain a constant level of fuel in the bowl. Fuel flows between theneedle valve and the valve seat and enters the float chamber. As the fuel enters the float chamber, thefloat moves upward to the pre-determined level becauseof the action of buoyancy. When the buoyancy and thefuel pressure balance, the needle valve and the valvedraw close to each other, shutting off the supply of fuelfrom the pump.The fuel level in the bowl controls the amount of fuel inthe fuel mixture. Too high a level allows more fuel thannecessary to leave the nozzle, enriching the mixture.Too low a level results in a leaner mixture, as notenough fuel leaves the nozzle. Therefore, the predetermined fuel level should not be changed arbitrarily.3-4.Starting device (starter system) (Fig. 9)Fig. 8In place of the choke, the starter systemis employed for Mikuni carburetors. In thestarter type, fuel and air for starting theengine are metered with entirely independentjets. The fuel metered in the starter jet ismixed with air and is broken into tinyparticles in the emulsion tube. The mixturethen flows into the plunger area, mixes againwith air coming from the air intake port forstarting and is delivered to the engine in theoptimum air-fuel ratio through the fuel discharge nozzle. The starter is opened andclosed by means of the starter plunger. Sincethe starter type is constructed .so asto utilizethe negative pressure of the inlet pipe, it isimportant that the throttle valve is closed,Fig. 9-&-

when starting the engine. At high temperatures (approx. above 68 F), the engine can be started withoutusing the starter.4.Tuning up of the engine and selection of the aperture of carburetorTuning up normally means a process of accurate and careful adjustment to obtain maximum engineperformance, although it means in a broad sense an economical improvement in fuel consumption.Improvement of power output of the engine depends on the amount of air drawn into the cylinder perunit time. A practice generally followed for engine tune-up includes:(1)To improve suction efficiency and exhaust efficiency by remodelling the intake and exhaustsystem(2)To improve combustion efficiency by raising the compression ratio(3)To increase the number of revolutions by adjusting the ignition timingJust as we need to take balanced meals of high calories