MIKUNI VM CARBURETORSUPER TUNING .UAl nit: MIKUNI !!'M~K~!~!~ N~2!~E~RATION CALIFORNIA 91324 U.S.A. http://www.motorcycle.in.th EXPLODED VIEW :===i 3~ 31 ~ 3 4 30 1. Rubber Cap, Throttle Cable 2. Cable Adjuster 3. Locknut, Cable Adjuster 4. Top, M:l.xing Chamb er 5. Tube, Air Vent 6. Spring, Piston Valve 7. Plate, Needle Retainer 8. "E" Ring 9. Jet Needle 10. Piston Valve 11. Needle Jet 12. Rubber Cap, Starting System 13. Cap, Starter Plunger 14. Lever Assy, Starting System 15. Leaf Spring, Lever Positioning 16. Spring, Starter Plunger 17. Plunger, Starting System 18. Body, Mixing Chamber 19. Gasket, Float Chamber 20. Baffle Plate, Float Chamber 2l. Pilot Jet 22. Pin, Float Arm Hinge 23. Float Arm 24. Float 25. Float Chamber 26. Washer, Float Chamber Plug 27. Plug, Float Chamber 2B. Air Jet 29. Air Screw 30. Spring, Air Adjusting Screw 3l. Spring, Idle Adjusting Screw 32. Screw, Idle Adjusting 33. Washer, Needle & Seat Assy 34. Needle & Seat Assy 35. Cup, Fuel Retaining 36. Main Jet 37. Plate, Vent Tube Retaining 3B. Screw, Float Chamber 29 ~ 4 18 15 16 6 17 28 19 21 ,I 2 7 3 8 5 20 35 9 36 34 2 11 2 2 37 I 2 27 38 PARTS LIST 008-49 _0-44 Vll32-33 Vll34-20 VllJ6-4 _3 VI00-44 006-4 "44-3 1IM26/46 VH26/46 VH26/46 VM26/46 VH26/46 20. M8/"7 VH34/114 VM34/114 006/40 008-3 _6/40 _0-4 M6/46 VllJ2-33 _4/114 VllJ4-20 006/46 vw.0-4 VH26/46 008-49 1. _4/20 VH44/20 2. M21/14 HZ!/l4 M21/14 M21/14 M21/14 K21/14 1tt1/14 M21/14 21. VM22/210 VK22/210 VK22/210 VH.2.21210 VK22/210 VK22/210 VM22/210 VK22/210 3. !lO/247 830/247 B30/241 830/247 B30/247 B30/247 BJ.O/247 BlO/241 BV26/22 BV26/22 BV26/22 BV26/22 BV26/22 BV26/22 BV26/22 BV26/2:2 4. VK26!S6 VM34/27 VH34/21 YH34/27 VK36/45 VMJ6/4' VM44/0S VM44/05 22. 23. VH28/166 VK34173 VM34173 VM34173 VH36!15 VM36/1S VK36/15 VM36/15 5. VMl7/173 IIM28/78 VM28/78 VH28/78 VH28/78 IIM28/78 VM28178 IIM28/78 24. VM28/164 _4/61 VMl4/61 VM34/61 VlI241236 YM24/236 YM'4/236 VH24/236 6. 1IM24/195 004/50 _4/50 004155 004/55 004/55 _4/13 Vll44/13 25. 006/130 004/68 VK34/68 VM34/68 006/33 006/33 Vll44104 Vl!44/04 7. 8. 008/132 004/31 004/31 004/31 VM35/09 005/09 _4/09 Vll44/09 26. 0081134 008/254 _8/254 _8/254 008/254 IIM28/254 008/134 _8/134 1IM20/369 1IM201369 1IM20/369 IIM20/36. 000136. 000/369 8&32/126 BS32/126 YM28/253 '11<28/'53 VM28/25J 008/253 VM28/253 008/133 Vl!28/133 J8-11'6 27. 28. 1IM28/133 Ja-ip6 BS30/97 8530/91 _4-3 9. J8-5P21 J8-6F5 J8-6DP5 J'8-6DH3 J8-6!'J6 J8-6DPl BS30/97 8S)0/97 B530/97 B630/97 10. VK28/56 VM34/110 V!04/110 YM34/110 VM36/36 008/24. VM40/02 VM40/0Z 29. VK20/214 VK20/214 VMZo/214 VM20/214 VK20/214 VM20/214 VM20/214 VH20/2:14 VK34/05 VH34/0S YM34/05 VM34/05 VlO4/17 VH44!10 VM44/1D 30. Ml.2F/46 Kl2F/46 MllF/46 MI2l/46 Ml2F/46 Ml2l/46 Ml2F/46 M12F/46 B5)0/97 BS30/97 11. VK30/38 12. VK20/455 VM20/455 VM20/455 YH20/455 VK2.0/455 VH20/-45S VK20/li55 VM20/455 31. H20/11 M20/11 1tt0Ill M20111 M20/11 H20111 H20/ll 1tt0Ill 13. MD16/30 1IM20/4S6 IIM20/456 YlI20/456 IIM20/456 000/456 000/456 IIM20/456 32. VM24/224 VH28/142 '11<24/390 004/224 002/171 VM22/171 0041224 002/171 14. 008/4.7 1IM28/124 IIM28/l24 VH281124 VM36/58 006/58 006/58 006/58 33. YM26/25 B14/52 '34152 .34/52 834152 B34/52 .34/52 .34/52 15. IIM26/09 002/17 _2/17 VM32117 IIM26/0. VM26/0. YM'6/0' 1IM26/0. 34. VM26/26 VM34/39 VK34/39 VM34/39 VK34/39 VH34/39 VH34/3'9 VM34/39 16. VKl6/42 VMl6/42 VMl.6/42 YMHi/42 VMI6/42 VKl6/42 VMl6/42 VMl6/42 35. VM28/228 VM36/17 \I!06/11 VK)6/17 VM36117 VM38109 VK2B/228 VH28/22. 17. 18. VK26/116 VHl8/144 VMl8/144 YMl8/144 \'Kl8/144 VMI.8/144 VHl8/144 VMI8/144 36. 4/042 4/042 4/042 "/042 4/042 4/042 4/042 4/042 VK28SS YMJOSS VM32SS YM34SS VH36SS VM38SS VM40SS VH44SS 37. VKl5/164 VH28/79 VK28/19 VK2B/79 VK28/79 VK2B/19 IIM28/79 I •. 008/129 004/86 _4/86 VM34186 VM36/20 Y1t36/Z0 VM36/Z0 006/20 38. CWZ-0416 C112-0516 C112-0516 CII'-051& <>12-051& IIM28"9 C112-0516 CW2-0516 C112-0516 http://www.motorcycle.in.th .. r .....r----+--- Float Pilot jet Needle jet I I Starter jet : : Main jet I : I ~_L_-~ ~ ~ ===~ _-_-_-~:= ____-_-_-~ ===~ ======= === ______-_-=-==== ==~J : c::::..in....-..Foreword This manual is intended as a guide for users of Mikuni carburetors who want to learn the adjusting method to the best performance from our products..-...........-...--... I I I I I I I I iii I I I Jet needle I Starter plunger Throttle valve I I I By-pass ~! Pilot outlet Air jet I I L I ___ _ -I~~~"..........motorcycle.-. Fuel ¢JAir "Mixture Mounting angle fore and aft inclination of the carb should not exceed approx 20 degrees from horizontal..th -Ring .... fuel and an air-fuel mixture flows.......-... ~ r -..> Air ~Fuel "'Mixture http://www.--. special tuning of the engine is now The arrows that appear in the drawings in this text show the direction considered a routine practice.......-. In motorcycles..----............ in which air.. respectively.-.. . . . . . . . . . . . .. . . . . 1 2. .. . . . ... . 6 4-1. . . .. ... . . . .. . . What is a carburetor? . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . .. . . 13 7-1. .. . 3 3-2-1. . . . . . 1 3-1. . . Bleed type . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .10 -11 5-4. . .. ... . ... . . 11 6.. . . . .. . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3-4. . . . . .. . . . . . . . . . . . . . . . .. . . . . Carburetor setting . . . . .. . . . . . . . . . . . . .. . .. . . . . . Function and construction . . . . . . . . .. 5-6 4.. . . . .. . . . . . . . . . Air-fuel mixture ratio . . . . . . . . . . .. . .. . . . . . . . . .. . . . . . . . . . Primary type . .. . . . . . . . . .. . . :. . . .. . . . . . . .. . . . . .. . . . . . . . . . .. . . . .. . .. . . . . ... .. . . . . 1 2 7. . Selection of aperture of carburetor 4-2. . . .. . . . . . .. . . . . . . Pilot jet and the slow system . . .8 5-2.. . . . . . . . .. . . . .7 . . The cutaway size of throttle valve . . . . . . . . . . . .. .. . . . . . . . . Selection of needle jet and jet needle . . . .. .in... .. . . . . . .. . .. . .. . . . . . . .th . . . .. .. . . . . . . . . . . . . .. 13 http://www. . . . . . . . . . .. . .. . . . . 6 . . . .. .. .. . .•. . . Tuning up of engine and selection of aperture of carburetor . . . . . . . .. . . . . . . . . .. . . . . . .. . . . . . . . . . . . . . . .. . . . . . . .. . Float system .. . . . . . . . . . . . . . .. . . Main fuel system (main system) . . . . . . . . . .. . . ..7 . . . . . . 2-3 3-2. . . .. .. . . .. .. .. . . . . 9 -10 5-3. . 4 3-2-2. . . . . . . .. . . . . .4 3-3. . . . . . . .. . . . Low-speed fuel system (pilot system) . . ..7 5-1. . . . . . . . . .. . . . . . . . .. The amount of incoming air in relation to meteorological conditions . . .. . .TABLE OF CONTENTS 1. . . . . Tuning up of carburetor for racing .. . . . . . . . . . . . . . . . . . . . . Maintenance . .. . . . . . . . . . . . . . . . Sizes of Mikuni carburetors 5. . . . . .. . . 1 3. . . . . . . . . . . . . . .. .. . . . . . . . . . . . . . . . ... . . . . .. Starting device (starter system) . . . .. . . . . . . . .. . . . .. . Selection of main jet . . . . .. . .. . .motorcycle. . . . . the ratio of about 12 to 13 grams of air for Air-fuel mixture 10:1 Maximum power mixture ratio 1 gram of fuel produces the maximum output. a low-speed fuel system (the pilot system) and a main fuel system (the main system) are provided in Mikuni VM-type carburetors. by breaking fuel into tiny particles (in the form of vapor) and by mixing the fuel with air in a proper ratio. . of the engine. Theoretically. depending on operating conditions. a somewhat richer mixture (10 to 12 grams of air against 1 gram of fuel) may be 16:1 18:1 Theoretical mixture ratio required to prevent seizure of the engine.""" In reality. varying mixture ratios are required for the engine.in. -1http://www.1. 20 40 60 80 Throttle valve opening (%) 100 Fig. . etc. 1 is required for ordinary engines.motorcycle. Function of a carburetor The function of a carburetor is to produce combustible air-fuel mixture. . 1 3. . . Although the required mixture ratio varies more or less with the type. and to deliver the mixture to the engine. the mixture ratio shown in Fig. . Functions and construction The engine of a vehicle is operated under a wide range of conditions. 12:1. from idling with the throttle valve (1) remaining almost closed to the full load (the maximum output) with the throttle valve fully opened. . 2. A proper ratio (mixture ratio or air-fuel ratio) means an ideal air-fuel mixture that can burn without leaving an excess of fuel or air. . In order to meet the requirements for the proper mixture ratio under these varying conditions. . However. Whether the proper mixture ratio is maintained or not holds the key to the efficient engine operation. . the amount of air required for complete combustion of 1 gram of fuel under normal conditions is: gramsofoffuel air MI'xture ratl'o = 15 1 gram th eoret'lea I mix ' ture ra t'10 ". the cooling efficiency. . In the high speed range.. in the case of an engine with low 14:1 cooling efficiency. 1) The ratio of a mixture of fuel and air is called the mixture ratio or the air-fuel ratio and is generally expressed by the weight proportion. Air-fuel mixture required by the engine (Fig.th . . The fuel mixed with air in the optimum ratio at this stage then goes into the engine.c _iJ Fig.th . When the degree of the throttle valve opening is small at idling.. II I I II ~ ~G)--. 3 Fig.3-1.motorcycle. the pilot outlet alone cannot supply the required fuel and the shortage has to be made up with fuel injected from the bypass. There is also a one-hole type low speed fuel system -2http://www. 2 and Fig.in. a negative p~essure strong enough to draw fuel from the needle jet in the main fuel system is not created. the velocity of air flowing through the needle jet (2) is slow. The adjustment of the mixture ratio during this stage is made by the pilot jet and the air screw. in the case of a two-hole type fuel system. When the throttle valve opening gets wider for low speed operation. The fuel supply during this low speed operation is controlled by means of the pilot outlet (3) and the bypass (4) that are provided close to the engine. Low-speed fuel system (the pilot system) . 3 Since the engine is operated with the throttle valve almost closed at idling or in the low speed range. Consequently. Fig. 2 The mixture is again mixed with air coming from the bypass and is drawn into the pilot outlet to mix with air flowing through the main bore (7). fuel metered with the pilot jet (5) is mixed with air adjusted in a proper amount by the air screw (6) and is broken into fine particles. mainly used for carburetors having a small main bore. The process of producing the air-fuel mixture and of adjusting the mixture ratio are the same as in a two-hole type low speed system. Throttle valve opening (%)-- Fig. 5 -3http://www. 3-2. On Mikuni VM-type carburetors. the pilot system and the main system Fig. 5.th . 4 are of independent construction. There are two types of the main fuel system. The fuel flow in these two systems is shown in Fig.motorcycle. one is a primary Total amount of fuel flow '" type used widely for 2-cycle engines and the other is a bleed type which may be used for 4-cycle engines as " Main fuel system well as for rotary valve type 2-cycle Pilot fuel system / engines.in. 3-2-2. 7) The construction of the bleed-type main fuel system is the same as that of the primary type. fuel is chiefly metered with the main jet.in. after mixing with air flowing through the main bore.motorcycle. after being metered in the clearance between the needle jet and the jet needle. In the case of the primary type. 6 Bleed type (Fig. is designed to hold air in the body section of the needle jet and then to have the air and fuel metered. air that comes from the air jet is mixed with fuel that is metered with the needle jet and the jet needle. When the opening of throttle valve is between about a quarter and three quarters. it is mixed with air that is metered in the air jet and atomization of the fuel is accel erated. Fig. thereby regulating the amount of fuel that is injected to the engine. The mixture is then injected. When the throttle valve is opened more than three quarters for high speed operation.th . on the other hand. Primary type When the throttle valve is opened about 1/4 or more.3-2-1. the cutaway of the throttle valve serves to control the negative pressure on the needle jet. 7 -4http://www. 7). to the engine in the optimum air·fuel ratio. the velocity of air flowing through the needle jet increases and also the negative pressure increases to the point where fuel can be sucked in. During this process of operation. The bleed type. except for the bleed hole that is provided in the needle jet (Fig. fuel passes through the main jet and. Fig. Starting device (starter system) (Fig. fuel and air for starting the engine are metered with entirely independent jets.in. as not enough fuel leaves the nozzle. Since the starter type is constructed . Fuel flows between the needle valve and the valve seat and enters the float chamber. When the buoyancy and the fuel pressure balance. mixes again with air coming from the air intake port for starting and is delivered to the engine in the optimum air·fuel ratio through the fuel dis· charge nozzle.th . Therefore. The starter is opened and closed by means of the starter plunger. In the starter type. Too low a level results in a leaner mixture. 3·4. Fig. 8) The float system serves to maintain a constant level of fuel in the bowl. shutting off the supply of fuel from the pump. The fuel metered in the starter jet is mixed with air and is broken into tiny particles in the emulsion tube.3·3. the float moves upward to the pre·determined level because of the action of buoyancy.motorcycle. Too high a level allows more fuel than necessary to leave the nozzle. the needle valve and the valve draw close to each other. The fuel level in the bowl controls the amount of fuel in the fuel mixture. As the fuel enters the float chamber. enriching the mixture. the starter system is employed for Mikuni carburetors. 9) Fig. Float system (Fig. 9 -&http://www.so as to utilize the negative pressure of the inlet pipe. 8 I n place of the choke. it is important that the throttle valve is closed. The mixture then flows into the plunger area. the pre· determined fuel level should not be changed arbitrarily. th . a large aperture alone does not improve the output necessarily. on the other hand. it is true that a large aperture improves the power output in the high speed range. Improvement of power output of the engine depends on the amount of air drawn into the cylinder per unit time.in. (4) the driver's preference. the engine cylinders need to take sufficient air and fuel mixed in proper amounts. Selection of the aperture of carburetor One of the prerequisites for improving the output is to use a carburetor with as large an aperture as possible. 11 shows the values which we have obtained through our experience over the years. above 68°F). At high temperatures (approx. (2) the design of the engine. the output rather drops. These factors include (1) whether the vehicle is intended for racing. -6http://www. the maximum output. Fig. However. (3) driving technique of the driver. the engine can be started without using the starter. 4. The aperture of a carburetor is determined by various factors. although it means in a broad sense an economical improvement in fuel consumption. The function of a carburetor is to prepare and supply a mixture of fuel vapor and air to the engine cylinders in proper proportion for efficient combustion. Since the engine comes in a wide variety of types. 4-1.when starting the engine.motorcycle. In addition. As shown in Fig. etc. Tuning up of the engine and selection of the aperture of carburetor Tuning up normally means a process of accurate and careful adjustment to obtain maximum engine performance.11 should be taken only as reference values. In the slow speed range. 10. the maximum torque and the minimum number of revolutions for stable engine operation must also be taken into account. A practice generally followed for engine tune·up includes: (1) To improve suction efficiency and exhaust efficiency by remodelling the intake and exhaust system (2) To imprpve combustion efficiency by raising the compression ratio (3) To incrlJase the number of revolutions by adjusting the ignition timing Just as we need to take balanced meals of high calories in our daily life. the values given in Fig. 49 Output I Too large an aperture ______ I Proper ps I aperture~ /1 /I ~ // V ~ />-. Mikuni VM-type carburetors come in various sizes. Revolutions per minute (Rpm) Fig. For further information. the main jet is installed in such a manner as to permit quick and easy replacement from the outside by merely removing the jet holder. The body of carburetors is made of aluminum ')r zinc. The altitude (atmospheric pressure).motorcycle. a test (normally referred to as setting or match- ing) to select the proper jet or setting part should be made. For racing. it is best to ":etermine the proper size of the jet on the racing course. The size of the jet is determined by measuring the output in a bench or in a chassis dynamo test. 11 Sizes of Mikuni carburetors 4-2.th . because the following points must be taken into account.86 22 Q) Co « 0/ . a. with the main bore ranging from 39"(10) to 1.--0- 38 1.18 -e 1. 10 Fig.02 30 :::l Too small an aperture '" u 26 '0 ~ :::l t . see our catalog.34 34 ~ 8~ 1.73"(44) (in even numbers for the most part).~ --= . Carburetor setting Once the aperture of the carburetor is determined. In some carburetors. temperature and humidity of the racing course b.1.70 18 .in. The operation of the engine based on the topography of the racing course -7http://www.55 14 / (Inch) (mm) I 6 10 20 30 40 50 PS Per Cylinder Note: These are merely reference values on the basis of one cylinder and one carburetor. 5. up to No.•. 50.. 195 (in mUltiples of 5) No. (2) Sustained operation at low speeds and at heavy engine load (3) Sustained operation at high speeds (without knocking or seizure) As shown in Photo 1. select the engine speed according to the dimensions of the test course. change in road conditions. These models come in the following sizes. are: (1) Smooth and steady operation of the engine at as high a speed as possible under varying operating conditions such as shifting of the gear..motorcycle. Model A.•. by using different sizes of main jets. 95 (in multiples of 5) No. In this case. 50. on the racing course. etc..in. H (40-50 km/hl. 210. 120 ••. there are three types of main jet. among others. (3) Check the exhaust fumes and read the spark plug (Selection of the spark plug should be made based on the thermal value that would best suit power output of the engine).. 110.55. up to No. 60 •. up to No. 220 .. 55. Next. 500 (in multiples of 10) Model C No. Model A & B No. (2) Compare the gain in speed that you can obtain by quick acceleration from a constant speed of 25-30 M· p. The points to be checked. 100. 300 (in multiples of 10) -8http://www. (1) Select the largest main jet (the limit of a rich mixture) which affords the maximum revolutions per minute (the maximum speed). Band C. compare. 200. do the following on a flat road.5-1. Selection of the main jet First.ascending and descending slopes.• up to No.60 .th . the test results you obtained from the above..•. In each model of main jet. (4/042) (M10/14) " (N 100. 12 C Selection of the needle jet and the jet needle A carburetor with a piston-type throttle valve is also called a variable venturi-type carburetor.::: a.th opening (%) . 13 Fig. 14 -9http://www. +2-3(%) -2-3(%) ~ 0 .5 50 75 100 Throttle valve opening (%)_ A 5-2.r-2 -3 ~4 1 4 2 -3-6% 15 50 75 100(%) 100 Throttle valve opening Throttle valve Fig. Inthis type of carburetor.604) LL.in. +20-22(%) +8-10(%) +10-12(%) The straight line is for Model C main jet and the +4-5(%) -4-5(%) -8-10(%) -20-22(%) dotted line for Model A and B main jets...5 47. 61. the needle jet needle serve to control a proper air-fuel mixture ratio jet and the Bleed type Primary type . B Fig. 12 shows comparison of fuel flow curves.Fig. different sizes within the ~ range of ± 100A> were tried.motorcycle. th .'-:.... the pilot jet is too small.motorcycle.. on the other hand.0 r- _. Adjust the throttle stop screw to bring down the engine revolution to your target speed for idling.----- -I '" -. Too big a pilot jet.during the so-called medium throttle valve opening (between 1/4 and 3/4 opening). would give rise to heavy exhaust smoke as well as a dull exhaust noise.. turn the air screw left and right (between 1/4 and 1/2 turn) and select the position where the engine revolution reaches the maximum.. If you cannot maintain the speed in the range of 12~25 M·P·H (30---40 km/h) with the grip held constant._. 5-3.0 Air screw opening Best opning range Air screw opening Fig.- I 1\1 I 1---. The air-fuel mixture ratio is controlled by the height of liE" ring that is inserted into one of the five slots provided in the head of the jet needle.::>o'" "0 -.--'------' 3. 15 & 16) In the slow system of the carburetor. I i ---1 . --r----t--. the pilot outlet and the bypass have holes whose size matches the aperture provided to the body.in. the selection of the opening of the pilot jet and the air screw is important. Turn the grip a little at no-load operation and see if the engine revolution increasfs smoothly._ . select once more the position where the engine ! I . q ~V V iii 1---- c----- Vij cr: - ~j \ --\ --- ~V II / i--- --- \ 1-----. If the pilot jet is too small..- _. The jet needle tapers off at one end and the clearance between the jet needle and the needle jet increases as the throttle valve opening gets wider. warm up the engine adequately and set the throttle stop screw so that the engine revolution at idling will be about 10-20% higher than the number of revolutions you are aiming at. Hence..:.---'------'--. increase in the engine speed is slow and irregular. Pilot jet and the slow fuel system (Fig.----- " ---- "'" l'--r-.-1_~ 1. 13. The variation of fuel flow based on the height of "E" ring is shown in Fig. First. 15 -10http://www. 16 Fig. Whether or not the right size of these parts is used gives a big bearing on the engine performance at partial load. After this adjustment of the throttle stop screw is made. Then. Selection of the opening of the air screw should be made in the following manner.. On the other hand. If the air screw is opened over 3. the smaller the size of the cutaway.in. 17) The size of the cutaway of the throttle valve affects the air-fuel mixture ratio when the degree of the throttle valve opening is between 1/8 and 1/2. 17 75 100 . it would be better for acceleration to select 1-1/2 turn. Interchange of the cutaway is made. by turning the air screw left and right (between 1/4 and 1/2 alternately). The cutaway size of the throttle valve (Fig.th 25 50 Throttle valve Fig. 5-4. Fig. (2) To determine the "fully closed" position of the air screw.0. turn the air screw slightly. attention should be paid to the following points. resulting in a lean mixture.0 turn). air inflow resistance is reduced and causes the amount of air intake to increase.revolution reaches the maximum. The maximum number of turns in the opening of the air screw must be limited to 3.0 turn. At this point. especially in the range of 1/8 and 1/4 opening. with the throttle valve opening kept unchanged. the richer the air-fuel mixture will become. 17 shows the fuel flow curve in relation to the size of the cutaway. Fig. As the cutaway gets larger in size. The position where the air screw comes to a stop should be considered the "fully closed" position. 16 shows the fuel flow curve in relation to the opening of the air screw. when the low speed fuel system is out of balance with the main fuel system. the number of revolutions does not change in the range of 1-1/2 to 2.motorcycle. 5 15 -11- http://www. Excessive tightening of the air screw would damage the seat. the spring will not work and the air screw can come off during operation of the vehicle. (1) If there is a certain range in the opening of the air screw where the fast engine revolution can be obtained (for instance. Maintenance A carburetor consists of various precision·machined parts such as jets. (3) For carburetors whose main jet can be replaced from the outside.:.~1f(Rib)O.66-..74 .66 .66-.""1 ~:~ ~ Independent float type Model H (Inch) (mm) VM26 -74 VM28 -49 VM30 -44 VM32 -33 VM34 -20 VM36 -4 VM38 -3 VM40 -1 VM44 -1 .th . Handle each part carefully to avoid scratches.. bending..59-.66-.11) +-~i~ L_ j I I . 18) r~===l==t= i .94 .59-. etc. 18 -12http://www.86-.3(o..6. (2) Wash the jets and the carburetor proper in solvent and blow them out with compressed air. care should be exercised. J: . . Therefore.3(o.94 . When you fit the "0" ring.-1 l..in. apply a little lubricant or fuel to the "0" ring.86-. (L_J ~ r r-. an "0" ring is used to prevent leakage of fuel.94 . (1) Proper tools should be used for disassembling and reassembling of jets.74 .J4( Rib)O. when removing jets or disassembling the carburetor for cleaning.motorcycle.. (4) It is important to maintain the fuel level in the carburetor. If the float arm is bent accidentally.74 15-17 15-17 22-24 22-24 22-24 17-19 17-19 17-19 17-19 Fig.. adjust the height of rib to the specified measurement (refer to Fig. Do not touch the float arm.a..11) t-=::.66-.86-.66 .74 . when disassembling the carburetor. The amount of air in question decreases in proportion to a rise in elevation as shown in Fig. humidity. with the result that the power output drops markedly. Tuning up of carburetor for racing The maximum output of the-engine depends on: (1) The amount of air drawn into the cylinders (2) Whether an air-fuel mixture is delivered to the cylinders in a proper ratio Since the amount of air that is drawn into the carburetor varies with the temperature. In the case of the engine for racing where the maximum output is constantly called for. the atmospheric pressure... Suppose that the amount of air sucked into the cylinders at an elevation of zero is taken as 100 (The temperature and humidity in this case are considered constant)..7.. 19 o 10 32 50 68 86 Temperature 20 Fig. it is best to tune up the engine by making a matching test of the carburetor in accordance with the temperature and other conditions on the racing course.in.. that the fuel flow be adjusted in accordance with the altitude of the racing course and meteorological conditions prevailing at a given time. etc.::: 90 80 :. The amount of the incoming air in relation to meteorological conditions The amount of air drawn into the cylinders is influenced by such factors as the altitude. therefore.th 30 - . 7-1. the humidity. o . It is important. etc. 19. f:: 100 t ~ 80 ~ ~ .motorcycle. Reduction in the amount of air drawn into the cylinders changes the air-fuel mixture ratio. ~ o 500 1000 1500 2000 Altitude (m) Fig.::: ~ . the mixture ratio is also changed. the temperature.: :. 20 shows the relations between a rise in temperature and the amount of air drawn into the cylinders (In this case. the atmospheric pressure (elevation) and the humidity are considered unchanged and the amount of air going into the cylinders at 32°F (O°C) is taken as 100). Fig. 20 -13http://www. (b) The condition grows worse. These can be traced to two causes as a whole.th . (c) The condition grows worse. (1) (2) When an air-fuel mixture is too rich: (a) The engine noise is dull and intermittent. when the starter is opened. when the starter is opened. (c) Acceleration is poor. (e) The revolution of the engine fluctuates and lack of power is noticed. (d) Removal of the air cleaner will improve the condition somewhat. when the engine gets hot.Engine Trouble-shooting When the carburetor setting is not correct for the engine. (f) Spark plug is fouled.in. -14http://www. (e) Exhaust gases are heavy. (b) The condition improves. various irregularities are noticed. (d) Spark plug burns. When an air-fuel mixture is too lean: (a) The engine gets overheated.motorcycle. 5 15 52 VM40'44 10 http://www.4 83.5 35 VM26·28 I-VIDO-200-1 I-VM32-200-1 57.5 10.4 79.5 23 74.5 57.4 80.0 15 43 VM36·38 I-VM38-20Q-1 74.5 76 76 76 79 81 94 94 L 54 67 69 69 73 73 86 88 Throttle adjuster Left Left Left Left Left Right Left Left Right Right Right Right Right Right Right 550 800 770 750 850 830 1020 980 Ai Ai At Ai Ai Ai Ai Ai Air screw Right Weight (gr) Material RUBBER FLANGE DIMENSIONS (Millimeters) ~ a b c d e f g h i Remarks I-VM28-20Q-1 60 8.5 58 31 37 VM26·28 8.8 48 67 67 67 67 G 9 9 9 9 9 9 9 9 H 8 8 8 8 8 8 8 8 I 44 57.th .4 79.2 44.5 49 49 49 49 F 44 48 57.6 8.5 60 54 35 37 32 25.6 8.5 27.5 10 54 39.5 10.5 68 47 46 32 15 52 VM40'44 I-VM44-200-1 84 8.motorcycle.6 60 10 10 15 40 VM3O'V34 I-VM36-200-1 70 8.0 15 18 40 VIDO'V34 I-VM34-200-1 30 34 35.5 44.2 8.5 62 62 70 70 J 72 83.in.5 38.4 83.5 25.5 8.5 10 60 38 38 28 15 43 VM36·38 I-VM40-200-1 74.5 17.CARBURETOR DIMENSIONS ~ Dimension (mm) Position of Throttle adjuster and Air screw VM28-49 VM30-44 VM32-33 VM34-20 VM36-4 VM38-3 VM40-4 VM44-3 A 35 40 40 40 43 43 52 52 B 33 38 38 41 41 41 50 50 C 10 8 8 8 8 8 8 8 D 4 4 4 4 4 4 4 4 E 49 44.5 57.4 80.5 67 47 46 25.5 10 54 31 29.4 K 66. ..in.-_ _ _----l nl~ MIKUNI AMERICAN CORPORATION ADDRESS: 8910 MIKUNI AVENUE NORTHRIDGE.A.S. CALlFORNrA 91324 U.th .motorcycle. http://www.Functional Range EHectiveness of Tuning Parts in relation to the Throttle Valve Opening (Approximation) ThrOHle Valve Opening Full Open- ~ ~ ~ ~ Closed Pilot Air Screw & Jet Throttle Valve -------' Jet Needle -------' Needle Jet--------' Main Jet ..-_ _ _---1 Air Jet .