MIKUNI VM CARBURETORSUPER TUNING .UAl

nil: MIKUNI ~!:'M~K~!~~~N~2~~E~RATIONCALIFORNIA 91324 U.S.A.

EXPLODED VIEW

:===i1. Rubber Cap, Throttle Cable

30 29 2. Cable Adjuster3. Locknut,. Cable Adjuster4. Top, M:l.xing Chamber5. Tube, Air Vent

31 6. Spring, Piston Valve

3

~7. Plate, Needle Retainer8. "E" Ring

18 9. Jet Needle4 15 10. Piston Valve11. Needle Jet12. Rubber Cap, Starting System

16 13. Cap, Starter Plunger

6 14. Lever Assy, Starting System15. Leaf Spring, Lever Positioning

28 17 16. Spring, Starter Plunger19 17. Plunger, Starting System

18. Body, Mixing Chamber

2119. Gasket, Float Chamber20. Baffle Plate, Flost Chamber

72 21- Pilot Jet3 5 22. Pin, Float Arm Hinge8 23. Float Arm

9 20 24. Float25. Float Chamber

35 26. Washer, Float Chamber Plug27. Plug, Float Chamber28. Air Jet

36 29. Air Screw

34 30. Spring, Air Adjusting Screw31. Spring, Idle Adjusting Screw32. Screw, Idle Adjusting

2 33. Washer, Needle & Seat Assy34. Needle & Seat Assy35. Cup, Fuel Retaining36. Main Jet

112 37. Plate, Vent Tube Retaining

38. Screw, Float Chamber

2

372 I 3827

PARTS LIST008-49 VH30-44 VIl3Z-33 VIl34-Z0 Vll36-4 Vll38-3 YlII<0-4 _4-3

1. 006/46 M6/46 1IM26/46 VH26/46 VH26/46 VM26/46 VH26/46 VH26/46

2. M21/14 HZ!/l4 M21/14 M21/14 M21/14 K21/14 1<21/14 M21/14

3. !J0/247 830/247 B30!241 830/247 B30/247 .30/247 BJ.O/247 B30!241

4. VK26/56 VM34/27 VH34/21 YH34/27 VK36/45 VMJ6/4' VM44!OS VM44!05

5. VMl7/173 1IM28/78 VM28/78 VH28178 VH28/78 VM28178 VM28178 VH28178

6. 1IM24/195 Vll34/50 VH34/50 004155 004155 004155 _4/13 VM44111

7. 008/132 004/31 VH34131 004131 VMS5109 005109 _4/09 OO41D9

8. IIM2D/369 0001369 1IM20/369 1IM201369 1III2DI369 0001369 8&32/126 BS32/126.. J8-5P21 J8-6]'5 J8-6DP5 J'S-6Oft3 J8-6!'J6 J8-6DPl JS-iF6 J8-11'6

10. VK28/S6 \lM34!110 V!04/110 YM34/110 VM36/36 008/24. VM40/02 VM40/0Z

11. VK30/38 VK34!OS. VH34/05 YM34/0S VM34/0S VM34/17 VH44/10 VM44/1D

12. VK20/4S5 VM20}455 VM20/455 ¥H20/4SS VK2.D/455 VH20/4SS VK2D/liS5 VM20/455

13. MD16/3D IIM2D/456 IIM2DI4S6 "Z01456 1IM201456 VIl2DI456 IIM2DI456 VH2014S6

14. VH28/497 1IM28/1Z4 1IM281124 VH281124 VMS6158 006158 VH36/58 1'>126158

15. 006/0. VI02/17 VI02/17 VI02117 1IM26109 VH26/09 "26/09 1IM26109

16. VKl6/42 VMl6/42 VMl.6/42 YMHi/42 VMl6/42 VKl6/42 VMl6/42 VMl6/42

17. VK26/116 VHl8/144 VMl8/144 YMl8/144 \'Kl8/144 VMI.8/144 VHl8f144 VMI8/144

18. VK28S5 YMJOSS \lM32SS YM345S VH36SS VM38SS VM40SS VH44SS

19. 0081129 004/86 004186 1IM34186 VMS6/ZD OO6/Z0 006/20 006120

008-49 VI00-44 Vll32-33 Vll34-Z0 Vll36-4 008-3 Yll40-4 "44-J

2D. MSI227 VM341114 VlO41114 Yll34/114 Yll36/40 VI06140 004120 VH44/Z0

21. VM22/210 VK22/210 VK22/210 VM2.2!210 VK22/210 VK22/210 VM22/210 VM22/210

22. BV26/22 BV26/22 BV26/22 BV26/22 BV26/22 BV26/22 BV26/22 BV26/2:2

23. VH28/166 VM34173 VM34173 VM34173 VH36!15 VM36/1S VK36/15 VM36/15

24. VM28/164 VlO4/61 VM34/61 Yll34161 004/236 ""241236 ""241236 VH24/Z36

25. VH281130 VlO4168 VK34168 Yll34168 006/33 VH36133 004104 VIl44/D4

26. 1IM281134 1III281Z54 1IM281254 0081254 008/254 1IM28/2S4 0081134 1IM28/U4

27. 1IM281133 0081253 111128/253 VM2S1253 VH28/Z53 VM28/253 0081133 VIl2S/UJ

28. BS30/97 8530/97 B530/97 B530/97 BS30/97 B630/97 B530/97 8530/91

29. VK20/214 VK20/214 VM20/214 VM20/214 VK20/214 VM20/214 VM20/214 VH20/Zl4

30. Ml.2F/46 Kl2F/46 MlIl/46 MI2l/46 Ml2l/4b Ml2l/46 Ml2F/46 M12F/46

31. H20111 .20/11 1<20111 .20111 .20111 .20111 .20/11 1<20111

32. VM241224 VH281142 004/390 0041224 002/171 VM22/171 0041224 002/171

33. VM26125 Bl4/52 834152 834152 83415Z 834/52 834/52 834152

34. VM26/26 VM34/39 VK34/39 VM34/39 VK34/39 VH34/39 VH3413'9 VM34/39

35. VM28/228 VM36/17 \I!06/11 VK36/17 VM36117 VM38/09 VK2B/228 VH2B/22.

36. 4/042 4/042 4/042 "/042 4/042 "/042 4/042 4/042

37. VKlS/164 VH28/79 VK28/19 VK2B/79 VK28/79 VH28/79 VK2B/79 1IM28/79

38. CWZ-0416 C112-0516 CIIZ-0516 CIIZ-0516 <>12-0516 OW2-D516 CWZ-D516 CIIZ-0516

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. In motorcycles, special tuning of the engine is now

considered a routine practice. The arrows that appear in the drawings in this text show the direction

in which air, fuel and an air-fuel mixture flows, respectively.

¢JAir Fuel "Mixture

Mounting angle fore and aft inclination of the carb should not exceed approx 20 degrees

from horizontal.

Pilot jet

Jet needle

-,

'''--Ring

Needle balve~

Air jet

IIIIII I I

~~~~:~~===~~:~~:=~~~-~~===~=============~-=======~

r- -- - --- -- -- -- - --- - ---~- ----- --- -- - - -- - - - - -----""]r -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -, II II II I

: ~!I UII II II I

I :I II I

: I

: Throttle valve :I I

~!

Float

c:::::;> Ai r ~Fuel "'Mixture

TABLE OF CONTENTS

1. What is a carburetor? .....••....•.••...•...•.•............•......•.... 1

2. Air-fuel mixture ratio ..••....•..•........•...••................•...... 1

3. Function and construction ...•..••............•............. 1

3-1. Low-speed fuel system (pilot system) ••.......•...•.................. 2-3

3-2. Main fuel system (main system) ••.........•.•.•.•.................... 3

3-2-1. Primary type .. . . . • . . . . . . . • . . . . . . . . . .........•.............4

3-2-2. Bleed type .••.......•....................................4

3-3. Float system ...•.......•............•...•......................5

3-4. Starting device (starter system) 5 ~6

4; Tuning up of engine and selection of aperture of carburetor .............•.....••...6

4-1. Selection of aperture of carburetor •..•............... :...... 6 - 7

4-2. Sizes of Mikuni carburetors 7

5. Carburetor setting 7

5-1. Selection of main jet .•.......................................•....8

5-2. Selection of needle jet and jet needle ...........•.......•........... 9 ~10

5-3. Pilot jet and the slow system ..•........................•........10-11

5-4. The cutaway size of throttle valve ..........................•......... 11

6. Maintenance •....•...................•........•.....•............. 12

7. Tuning up of carburetor for racing .........................•.............13

7-1. The amount of incoming air in relation to meteorological conditions ..•..•....... 13

1. Function of a carburetor

The function of a carburetor is to produce combustible air-fuel mixture, by breaking fuel into tiny

particles (in the form of vapor) and by mixing the fuel with air in a proper ratio, and to deliver the

mixture to the engine. A proper ratio (mixture ratio or air-fuel ratio) meansan ideal air-fuel mixture that

can burn without leaving an excess of fuel or air, Whether the proper mixture ratio is maintained or not

holds 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 generally

expressed by the weight proportion. Theoretically, the amount of air required for complete combustion

of 1 gram of fuel under normal conditions is:

MI'xt ure ratio = 15 grams of air th t' I . t ti1 gram of fuel "....... eore rca mix ure ra 10

In 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 the

engine, the mixture ratio shown in Fig. 1 is

Air-fuel mixture

Theoretical mixture ratio

Maximum power mixture ratio

18:1

10:1

12:1 .

16:1

14:1

required for ordinary engines. In the high speed

range, the ratio of about 12 to 13 grams of air for

1 gram of fuel produces the maximum output.

However, in the case of an engine with low

cooling efficiency, a somewhat richer mixture (10

to 12 grams of air against 1 gram of fuel) may be

required to prevent seizure of the engine.

3. Functions and construction

20 40 60 80 100Throttle valve opening (%)

Fig. 1

The engine of a vehicle is operated under a wide range of conditions, from idling with the throttle

valve (1) remaining almost closed to the full load (the maximum output) with the throttle valve fully

opened. In order to meet the requirements for the proper mixture ratio under these varying conditions, a

low-speed fuel system (the pilot system) and a main fuel system (the main system) are provided in

Mikuni VM-type carburetors.

-1-

3-1. Low-speed fuel system (the pilot system) ... Fig. 2 and Fig. 3

Since 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 negative p~essure strong

enough to draw fuel from the needle jet in the main fuel system is not created. 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. When the degree of the throttle valve opening is small at idling, 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.

Fig. 2 Fig. 3

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). The fuel mixed with air in the optimum ratio at this stage

then goes into the engine. When the throttle valve opening gets wider for low speed operation, the pilot

outlet alone cannot supply the required fuel and the shortage has to be made up with fuel injected from

the bypass. 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. There is also a one-hole type low speed fuel system

-2-

Fig. 4

Total amount of fuel flow

""

Pilot fuel system

/engines.

mainly used for carburetors having a

small main bore. The process of pro­

ducing the air-fuel mixture and of

adjusting the mixture ratio are the

same as in a two-hole type low speed

system.

3-2. On Mikuni VM-type carburetors,

the pilot system and the main system

are of independent construction.

The fuel flow in these two systems is

shown in Fig. 5. There are two types of

the main fuel system; one is a primary

type used widely for 2-cycle engines

and the other is a bleed type which

may be used for 4-cycle engines as

well as for rotary valve type 2-cycle

Throttle valve opening (%)-­

Fig. 5

-3-

3-2-1. Primary type

When the throttle valve is opened about 1/4 or more, 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. When the

opening of throttle valve is between about a quarter and three quarters, fuel passes through the main jet

and, after being metered in the clearance between the

needle jet and the jet needle, it is mixed with air that is

metered in the air jet and atomization of the fuel is

accelerated.

The mixture is then injected, after mixing with air flowing

through the main bore, to the engine in the optimum

air-fuel ratio. During this process of operation, the

cutaway of the throttle valve serves to control the negative

pressure on the needle jet, thereby regulating the amount

of fuel that is injected to the engine. When the throttle

valve is opened more than three quarters for high speed

operation, fuel is chiefly metered with the main jet.

3-2-2. Bleed type (Fig. 7)

The construction of the bleed-type main fuel system

is the same as that of the primary type, except for the

bleed hole that is provided in the needle jet (Fig. 7).

In the case of the primary type, air that comes from the air

jet is mixed with fuel that is metered with the needle jet

and the jet needle. The bleed type, on the other hand, is

designed to hold air in the body section of the needle jet

and then to have the air and fuel metered.

-4-

Fig. 6

Fig. 7

3-3. Float system (Fig. 8)

The float system serves to maintain a constant level of fuel in the bowl. Fuel flows between the

needle valve and the valve seat and enters the float chamber. As the fuel enters the float chamber, the

float moves upward to the pre-determined level because

of the action of buoyancy. When the buoyancy and the

fuel pressure balance, the needle valve and the valve

draw close to each other, shutting off the supply of fuel

from the pump.

The fuel level in the bowl controls the amount of fuel in

the fuel mixture. Too high a level allows more fuel than

necessary to leave the nozzle, enriching the mixture.

Too low a level results in a leaner mixture, as not

enough fuel leaves the nozzle. Therefore, the pre­

determined fuel level should not be changed arbitrarily.

3-4. Starting device (starter system) (Fig. 9)

In place of the choke, the starter system

is employed for Mikuni carburetors. In the

starter type, fuel and air 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 in the

optimum air-fuel ratio through the fuel dis­

charge nozzle. The starter is opened and

closed by means of the starter plunger. Since

the starter type is constructed .so asto utilize

the negative pressure of the inlet pipe, it is

important that the throttle valve is closed,

Fig. 8

Fig. 9

-&-

when starting the engine. At high temperatures (approx. above 68°F), the engine can be started without

using the starter.

4. 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, 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 per

unit time. 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 improve combustion efficiency by raising the compression ratio

(3) To increase 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 engine cylinders need to

take sufficient air and fuel mixed in proper amounts. 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.

4-1. 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. However, a large aperture alone does not improve the output necessarily. As shown in Fig. 10, it

is true that a large aperture improves the power output in the high speed range. In the slow speed range,

on the other hand, the output rather drops. The aperture of a carburetor is determined by various

factors. These factors include (1) whether the vehicle is intended for racing, (2) the design of the engine,

(3) driving technique of the driver, (4) the driver's preference, etc. In addition, the maximum output, the

maximum torque and the minimum number of revolutions for stable engine operation must also be taken

into account. Fig. 11 shows the values which we have obtained through our experience over the years.

Since the engine comes in a wide variety of types, the values given in Fig.11 should be taken only as refer­

ence values.

-6-

Fig. 10

Revolutions per minute (Rpm)

Output

Ips

----=-Too large an aperture______

~I I ---Proper aperture""", <-:

~ Too smallV an aperture

/1/;~

0/// /

I

1.49 38

1.34 34

~

8 1.18 30e:::l-e 1.02 26'"o'0e .86 22:::ltQ)c,« .70 18

.55 14

(Inch) (mm)

6 10 20 30 40 50PS Per Cylinder

Note: These are merely reference valueson the basis of one cylinder and onecarburetor.

Fig. 11

4-2. Sizesof Mikuni carburetors

Mikuni VM-type carburetors come in various sizes, with the main bore ranging from 39"(10) to

1.73"(44) (in even numbers for the most part). The body of carburetors is made of aluminum or zinc.

In some carburetors, the main jet is installed in such a manner asto permit quick and easy replacement

from the outside by merely removing the jet holder. For further information, see our catalog.

5. Carburetor setting

Once the aperture of the carburetor is determined, a test (normally referred to as setting or match­

ing) to select the proper jet or setting part should be made. The size of the jet is determined by measur­

ing the output in a bench or in a chassis dynamo test. For racing, it is best to ":etermine the proper size

of the jet on the racing course, because the following points must be taken into account.

a. The altitude (atmospheric pressure), temperature and humidity of the racing course

b. The operation of the engine based on the topography of the racing course

-7-

5-1. Selection of the main jet

First, do the following on a flat road.

(1) Select the largest main jet (the limit of a rich mixture) which affords the maximum revolutions

per minute (the maximum speed). In this case, select the engine speed according to the dimensions of the

test course.

(2) Compare the gain in speed that you can obtain by quick acceleration from a constant speed

of 25-30 M· p. H (40-50 krn/h), by using different sizes of main jets.

(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 enginel.

Next, compare, on the racing course, the test results you obtained from the above. The points to be

checked, among others, 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, change in road conditions,ascending and descending

slopes, etc.

(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, there are three types of main jet, Model A, Band C. These models come in

the following sizes.

Model A & B

No. 50,55,60 ..... up to No. 95 (in multiples of 5)

No. 100, 110, 120 ••.•. up to No. 500 (in multiples of 10)

Model C

No. 50, 55, 60 •.•.. up to No. 195 (in multiples of 5)

No. 200, 210, 220 .•..• up to No. 300 (in multiples of 10)

-8-

Throttle valve opening (%)_

Fig. 12

Fig. 12 shows comparison of fuel flow curves.

The straight line is for Model C main jet and the

dotted line for Model A and B main jets. In each

model of main jet, different sizes within the

range of ± 100A> were tried.

(4/042) (M10/14) (N 100.604)

I • .,A B C

61.5

47.5 50 75

+20-22(%)+S-10(%)

-S-10(%)-20-22(%)

100

5-2. 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. Inthis type of carburetor, the needle

jet and the jet needle serve to control a proper air-fuel mixture ratio

Sleed type

1..r-2-3~4

4

Primary type

15 50 75Throttle valve opening

Fig. 13

100(%)

-9-

15 25 50 100Throttle valve opening (%)

Fig. 14

during the so-called medium throttle valve opening (between 1/4 and 3/4 opening), Whether or not the

right size of these parts is used gives a big bearing on the engine performance at partial load. 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. The air-fuel mixture ratio is controlled by the height of "E" ring that is

inserted into one of the five slots provided in the head of the jet needle. The variation of fuel flow based

on the height of "E" ring is shown in Fig. 13.

5-3. Pilot jet and the slow fuel system (Fig. 15 & 16)

In the slow system of the carburetor, the pilot outlet and the bypass have holes whose size matches

the aperture provided to the body. Hence, 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 increases

smoothly. If the pilot jet is too small, increase in the engine speed is slow and irregular. Too big a pilot jet,

on the other hand, would give rise to heavy exhaust smoke aswell as a dull exhaust noise. If you cannot

maintain the speed in the range of 12~25 M·P·H (3cr-40 km/h) with the grip held constant, the pilot

jet is too small.

Selection of the opening of the air screw should be made in the following manner. First, 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. Then, 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.

Adjust the throttle stop screw to bring down the engine revolution to your target speed for idling. After

this adjustment of the throttle stop screw is made, select once more the position where the engine

1.0 3.0Air screw opening

Fig. 16

I I--

I_\1 . ---- --

.. _--- f\ -- ------__H __ --'_.

\ -1------- ---

--- _m

\\~.,-.

,._. • __•• M••

\------ '--- --. <m

-,,-- ..-

<,'---- -.,.....• .... I ..... n __ - ...- t--- .

_._.-l_~ _.

Best opning rangeAir screw opening

Fig. 15

! I, I

i---1---

---_._-

--I

i-~-7 -.....

/ /;1/ ~I

_.-

II1---- Vii'II---

~~- -

..eo..,::>'0iii

tI:

...!

-10-

revolution reaches the maximum, by turning the air screw left and right (between 1/4 and 1/2

alternately). At this point, attention should be paid to the following points.

(1) If there is a certain range in the opening of the air screw where the fast engine revolution can

be obtained (for instance, the number of revolutions does not change in the range of 1-1/2 to 2.0 turn),

it would be better for acceleration to select 1-1/2 turn.

(2) To determine the "fully closed" position of the air screw, turn the air screw slightly. Excessive

tightening of the air screw would damage the seat. The position where the air screw comes to a stop

should be considered the "fully closed" position. The maximum number of turns in the opening of the

air screw must be limited to 3.0. If the air screw is opened over 3.0 turn, the spring will not work and the

air screw can come off during operation of the vehicle. Fig. 16 shows the fuel flow curve in relation to

the opening of the air screw.

5-4. The cutaway size of the throttle valve (Fig. 17)

1007525 50Throttle valve

Fig. 17

The size of the cutaway of the throttle valve affects the air-fuel mixture ratio when the degreeof

the throttle valve opening is between 1/8 and 1/2, especially in the range of 1/8 and 1/4 opening. As the

cutaway gets larger in size, with the throttle

valve opening kept unchanged, air inflow

resistance is reduced and causes the amount

of air intake to increase, resulting in a lean

mixture. On the other hand, the smaller the

size of the cutaway, the richer the air-fuel

mixture will become. Interchange of the

cutaway is made, when the low speed fuel

system is out of balance with the main fuel

system.

Fig. 17 shows the fuel flow curve in relation

to the size of the cutaway.

-11-

6. Maintenance

A carburetor consists of various precision-machined parts such as jets. Therefore, care should be

exercised, when removing jets or disassembling the carburetor for cleaning.

(1) Proper tools should be used for disassembling and reassembling of jets. Handle each part

carefully to avoid scratches, bending, etc.

(2) Wash the jets and the carburetor proper in solvent and blow them out with compressed air.

(3) For carburetors whose main jet can be replaced from the outside, an "0" ring is used to

prevent leakage of fuel. When you fit the "0" ring, apply a little lubricant or fuel to the "0" ring.

(4) It is important to maintain the fuel level in the carburetor. Do not touch the float arm, when

disassembling the carburetor. If the float arm is bent accidentally, adjust the height of rib to the

specified measurement (refer to Fig. 18)

t-::::;~d:d( Rib)O.3(o.11)

i I, J:

.~1f(Rib)O.3(o.11)

+-~i~L--1 I r I

~ , (L_JL...._,"",\ r-.-J

~:~~

Independent float type

Model VM26 VM28 VM30 VM32 VM34 VM36 VM38 VM40 VM44-74 -49 -44 -33 -20 -4 -3 -1 -1

H(Inch) .59-.66 .59-.66 .86-.94 .86-.94 .86-.94 .66-.74 .66-.74 .66-.74 .66-.74(rnm) 15-17 15-17 22-24 22-24 22-24 17-19 17-19 17-19 17-19

Fig. 18

-12-

7. 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, the

atmospheric pressure, humidity, etc., the mixture ratio is also changed. It is important, therefore, that

the fuel flow be adjusted in accordance with the altitude of the racing course and meteorological

conditions prevailing at a given time.

7-1. 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, the

temperature, the humidity, etc. 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). The

amount of air in question decreases in proportion to a rise in elevation asshown in Fig. 19. Reduction in

the amount of air drawn into the cylinders changes the air-fuel mixture ratio, with the result that the

power output drops markedly. Fig. 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).

In the case of the engine for racing where the maximum output is constantly called for, 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.

f::~ 80

~:;:::....~

o 500 1000 1500 2000

Altitude (m)

Fig. 19

-13-

t10 0

90

~ 80so

:;:::....~

0 10 20 30 40 (oC)

32 50 68 86 104 (OF)

Temperature -Fig. 20

Engine Trouble-shooting

When the carburetor setting is not correct for the engine, various irregularities are noticed. These

can be traced to two causes as a whole.

(1) When an air-fuel mixture is too rich:

(a) The engine noise is dull and intermittent.

(b) The condition grows worse, when the starter is opened.

(c) The condition grows worse, when the engine gets hot.

(d) Removal of the air cleaner will improve the condition somewhat.

(e) Exhaust gases are heavy.

(f) Spark plug is fouled.

(2) When an air-fuel mixture is too lean:

(a) The engine gets overheated.

(b) The condition improves, when the starter is opened.

(c) Acceleration is poor.

(d) Spark plug burns.

(e) The revolution of the engine fluctuates and lack of power is noticed.

-14-

CARBURETOR DIMENSIONS

~ 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 44.2 44.5 49 49 49 49

Dimension F 44 48 57.8 48 67 67 67 67(mm)

G 9 9 9 9 9 9 9 9

H 8 8 8 8 8 8 8 8

I 44 57.5 57.5 57.5 62 62 70 70

J 72 83.4 83.4 83.4 80.4 80.4 79.4 79.4

K 66.5 76 76 76 79 81 94 94

L 54 67 69 69 73 73 86 88

Position of Throttle Left Left Left Left Left Right Left LeftThrottle adjusteradjusterandAir Bcrew Air sc rew Right Right Right Right Right Right Right Right

Weight (gr) 550 800 770 750 850 830 1020 980

Material ss. ss: At ss. ss: ss. Ai Ai

RUBBER FLANGE DIMENSIONS (Millimeters)

~ a b c d e f g h i Remarks

I-VM28-20Q-l 60 8.5 10 54 31 29.5 27.5 17.5 35 VM26·28

I-VIDO-200-1 57.2 8.5 10 58 31 30 23 15 37 VM26·28

I-VM32-200-1 74.6 8.5 10 60 35 34 32 18 . 40 VIDO'V34

I-VM34-200-1 60 8.5 10 54 37 35.5 25.0 15 40 VM3O'V34

I-VM36-200-1 70 8.5 10 54 39.5 38.5 25.0 15 43 VM36·38

I-VM38-20Q-l 74.6 8.5 10 60 38 38 28 15 43 VM36·38

I-VM40-200-1 74.6 8.5 10.5 68 47 46 32 15 52 VM40·44

I-VM44-200-1 84 8.5 10.5 67 47 46 25.5 15 52 VM40·44

Functional Range EHectiveness of Tuning Parts in relation to theThrottle Valve Opening (Approximation)

Throttle Valve Opening

Full Open-

~

~

~~Closed

Pilot Air Screw & JetThrottle Valve ------'

Jet Needle ---l

Needle Jet--------'Main Jet --- ---l

Air Jet ---- ----l

nl~MIKUNI AMERICAN CORPORATIONADDRESS: 8910 MIKUNI AVENUE NORTHRIDGE,

CALlFORNrA 91324 U.S.A.