MIKUNI VM CARBURETOR
SUPER TUNING .UAl
nil: MIKUNI ~!:'M~K~!~~~N~2~~E~RATION
CALIFORNIA 91324 U.S.A.
EXPLODED VIEW
:===i
31
~
3
4
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, Flost Chamber
21- Pilot Jet
22. Pin, Float Arm Hinge
23. Float Arm
24. Float
25. Float Chamber
26. Washer, Float Chamber Plug
27. Plug, Float Chamber
28. Air Jet
29. Air Screw
30. Spring, Air Adjusting Screw
31. 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
38. Screw, Float Chamber
30 29
18
15
16
6
17
28
19
21
2
7
3
8
5
20
35
9
36
34
2
11
2
2
37
I
2
27
38
PARTS LIST
008-49
VH30-44
VIl3Z-33
VIl34-Z0
Vll36-4
Vll38-3
YlII<0-4
_4-3
008-49
VI00-44
Vll32-33
Vll34-Z0
Vll36-4
008-3
Yll40-4
"44-J
1.
006/46
M6/46
1IM26/46
VH26/46
VH26/46
VM26/46
VH26/46
VH26/46
2D.
MSI227
VM341114
VlO41114
Yll34/114
Yll36/40
VI06140
004120
VH44/Z0
2.
M21/14
HZ!/l4
M21/14
M21/14
M21/14
K21/14
1<21/14
M21/14
21.
VM22/210
VK22/210
VK22/210
VM2.2!210
VK22/210
VK22/210
VM22/210
VM22/210
3.
!J0/247
830/247
B30!241
830/247
B30/247
.30/247
BJ.O/247
B30!241
22.
BV26/22
BV26/22
BV26/22
BV26/22
BV26/22
BV26/22
BV26/22
BV26/2:2
4.
VK26/56
VM34/27
VH34/21
YH34/27
VK36/45
VMJ6/4'
VM44!OS
VM44!05
23.
VH28/166
VM34173
VM34173
VM34173
VH36!15
VM36/1S
VK36/15
VM36/15
5.
VMl7/173
1IM28/78
VM28/78
VH28178
VH28/78
VM28178
VM28178
VH28178
24.
VM28/164
VlO4/61
VM34/61
Yll34161
004/236
""241236
""241236
VH24/Z36
6.
1IM24/195
Vll34/50
VH34/50
004155
004155
004155
_4/13
VM44111
25.
VH281130
VlO4168
VK34168
Yll34168
006/33
VH36133
004104
VIl44/D4
7.
008/132
004/31
VH34131
004131
VMS5109
005109
_4/09
OO41D9
26.
1IM281134
1III281Z54
1IM281254
0081254
008/254
1IM28/2S4
0081134
1IM28/U4
8.
IIM2D/369
0001369
1IM20/369
1IM201369
1III2DI369
0001369
8&32/126
BS32/126
27.
1IM281133
0081253
111128/253
VM2S1253
VH28/Z53
VM28/253
0081133
VIl2S/UJ
J8-5P21
J8-6]'5
J8-6DP5
J'S-6Oft3
J8-6!'J6
J8-6DPl
JS-iF6
J8-11'6
28.
BS30/97
8530/97
B530/97
B530/97
BS30/97
B630/97
B530/97
8530/91
..
10.
VK28/S6
\lM34!110
V!04/110
YM34/110
VM36/36
008/24.
VM40/02
VM40/0Z
29.
VK20/214
VK20/214
VM20/214
VM20/214
VK20/214
VM20/214
VM20/214
VH20/Zl4
11.
VK30/38
VK34!OS.
VH34/05
YM34/0S
VM34/0S
VM34/17
VH44/10
VM44/1D
30.
Ml.2F/46
Kl2F/46
MlIl/46
MI2l/46
Ml2l/4b
Ml2l/46
Ml2F/46
M12F/46
VM20/455
12.
VK20/4S5
VM20}455
VM20/455
¥H20/4SS
VK2.D/455
VH20/4SS
VK2D/liS5
31.
H20111
.20/11
1<20111
.20111
.20111
.20111
.20/11
1<20111
13.
MD16/3D
IIM2D/456
IIM2DI4S6
"Z01456
1IM201456
VIl2DI456
IIM2DI456
VH2014S6
32.
VM241224
VH281142
004/390
0041224
002/171
VM22/171
0041224
002/171
VH36/58
1'>126158
33.
VM26125
Bl4/52
834152
834152
83415Z
834/52
834/52
34.
VM26/26
VM34/39
VK34/39
VM34/39
VK34/39
VH34/39
VH3413'9
VM34/39
VM28/228
VM36/17
\I!06/11
VK36/17
VM36117
VM38/09
VK2B/228
VH2B/22.
4/042
"/042
4/042
4/042
14.
15.
16.
VH28/497
006/0.
VKl6/42
1IM281124
VH281124
VI02/17
VI02/17
VI02117
1IM26109
VH26/09
"26/09
1IM26109
VMl6/42
VMl.6/42
YMHi/42
VMl6/42
VKl6/42
VMl6/42
VMl6/42
35.
VMl8/144
YMl8/144
\'Kl8/144
VMI.8/144
VHl8f144
VMI8/144
36.
1IM28/1Z4
VMS6158
006158
834152
17.
VK26/116
VHl8/144
4/042
4/042
4/042
"/042
18.
VK28S5
YMJOSS
\lM32SS
YM345S
VH36SS
VM38SS
VM40SS
VH44SS
37.
VKlS/164
VH28/79
VK28/19
VK2B/79
VK28/79
VH28/79
VK2B/79
1IM28/79
004/86
004186
1IM34186
VMS6/ZD
OO6/Z0
006/20
006120
38.
CWZ-0416
C112-0516
CIIZ-0516
CIIZ-0516
<>12-0516
OW2-D516
CWZ-D516
CIIZ-0516
19.
0081129
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.
Fuel
¢JAir
"Mixture
Mounting angle fore and aft inclination of the carb should not exceed approx 20 degrees
from horizontal.
r- -- - --- -- -- -- - --- -
---~-
----- --- -- - - -- - - - - -----""]
r -- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -,
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Jet needle
I
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Throttle valve
:
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Air jet
Float
Needle
balve~
Pilot jet
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~~~~:~~===~~:~~:=~~~-~~===~=============~-=======~
c:::::;> Ai r
~Fuel
"'Mixture
'''--Ring
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
5 ~6
3-4. Starting device (starter system)
4;
Tuning up of engine and selection of aperture of carburetor . . . . . . . . . . . . . • . . . . . • • . . . 6
4-1. Selection of aperture of carburetor
5.
•..•...............
:...... 6 - 7
4-2. Sizes of Mikuni carburetors
7
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 • . . . . • . . . . . . . . . . . . . . . . . . . • . . . . . . . . • . . . . . • . . . . . . . . . . . . . 1 2
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) means an 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:
. t ure rati10
"....... th eoret'rcaI mix
gramsofoffuel
air
MI'xt ure ratio = 15
1 gram
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
required for ordinary engines. In the high speed
range, the ratio of about 12 to 13 grams of air for
Air-fuel mixture
10:1
1 gram of fuel produces the maximum output.
12:1
However, in the case of an engine with low
14:1
cooling efficiency, a somewhat richer mixture (10
to 12 grams of air against 1 gram of fuel) may be
Maximum power mixture ratio
.
16:1
Theoretical mixture ratio
18:1
required to prevent seizure of the engine.
20
40
60
80
Throttle valve opening (%)
100
Fig. 1
3.
Functions and construction
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. 3
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). 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-
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.
3-2. On Mikuni VM-type carburetors,
the pilot system and the main system
Fig. 4
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
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
well as for rotary valve type 2-cycle
Pilot fuel system
/
engines.
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.
Fig. 6
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.
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 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 predetermined fuel level should not be changed arbitrarily.
3-4.
Starting device (starter system) (Fig. 9)
Fig. 8
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 discharge 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. 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 reference values.
-6-
Output
I
Too large an aperture______
I
I
Proper aperture""",
ps
/1
/; ~
//
V
~
1.49
38
1.34
34
8e 1.18
-e 1.02
30
----=-
~
<-: ---
~
:::l
Too small
an aperture
o
'"
26
'0
e:::l
t
.86 22
Q)
c,
«
0/
.70
18
.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.
Revolutions per minute (Rpm)
Fig. 10
Fig. 11
Sizes of Mikuni carburetors
4-2.
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.
5.
For further information, see our catalog.
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 measuring 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-
Fig. 12 shows comparison of fuel flow curves.
+20-22(%)
+S-10(%)
The straight line is for Model C main jet and the
-S-10(%)
-20-22(%)
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
5-2.
B
61.5
47.5
50
75
100
Throttle valve opening (%)_
Fig. 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. Inthis type of carburetor, the needle
jet and the
jet needle serve to control a proper air-fuel mixture ratio
Sleed type
Primary type
..r- 2
-3
~4
1
15
4
50
75
Throttle valve opening
100(%)
15 25
50
Throttle valve
Fig. 13
Fig. 14
-9-
100
opening (%)
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 as well 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
!
I
,
I
I
i
--- 1 - - -
I
_\1
- - -_._-
--
..
I
.,
::>
'0
/;1
/
..
e
.o
-.....
7
...
!
Vii
tI:
~~
.
---- --
f\
--
\
.,.....•
---
------
-1--
__ H
~.,-.
\
--.
<m
-,
.... I
.....
n
__
'---- - ...- t--- .
_.
3.0
Fig. 16
-10-
• _ _ •• M••
-
Air screw opening
Fig. 15
,._.
,-- ..
<,
1.0
Best opning range
Air screw opening
--'_.
----- - - -
_m
_._.-l_~
-
__
\\
-------- '---
'II
-
_---
i-~_.-
~I
II
/
iii 1----
I
--
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)
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, 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.
25
50
Throttle valve
Fig. 17
-11-
75
100
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)
i,
I
J:
.~1f(Rib)O.3(o.11)
t-::::;~d:d( Rib)O.3(o.11)
+-~i~
1~ ,I
L--
I
(L_J
r
r-.-J
L...._,"",\
~:~
~
Independent float type
Model
H
(Inch)
(rnm)
VM26
-74
VM28
-49
VM30
-44
VM32
-33
VM34
-20
VM36
-4
VM38
-3
VM40
-1
VM44
-1
.59-.66 .59-.66 .86-.94 .86-.94 .86-.94 .66-.74 .66-.74 .66-.74 .66-.74
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 as shown 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::
10 0
t
~ 80
~
s
o
~
:;:::
....
90
80
:;:::
~
....
~
o
500
1000
1500 2000
0
Altitude (m)
32
Fig. 19
10
20
68
86
50
Temperature
Fig. 20
-13-
30
40 (oC)
-
104 (OF)
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)
(2)
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.
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
~
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
F
44
48
57.8
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
57.5
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
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
ss.
ss:
At
ss.
ss:
ss.
Ai
Ai
Dimension
(mm)
Position of
Throttle
adjuster
and
Air Bcrew
VM28-49
Air sc rew Right
Weight (gr)
Material
RUBBER FLANGE DIMENSIONS (Millimeters)
~
I-VM28-20Q-l
a
60
b
8.5
c
d
e
10
54
31
f
29.5
g
27.5
h
i
Remarks
17.5
35
VM26·28
37
VM26·28
I-VIDO-200-1
57.2
8.5
10
58
31
30
23
15
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 the
Throttle Valve Opening (Approximation)
Throttle Valve Opening
Full Open-
~
~
~
~
Closed
Pilot Air Screw & Jet
Throttle Valve ------'
Jet Needle
---l
Needle Jet--------'
Main Jet - - Air Jet - - - -
nl~ MIKUNI AMERICAN CORPORATION
ADDRESS: 8910 MIKUNI AVENUE NORTHRIDGE,
CALlFORNrA 91324 U.S.A.
---l
----l