1. Carburetor Theory

2. Variable Venturi or “slide”
carburetors
The goal of providing

the stochiometric
ratio, (14.7:1) is
tempered with the
goal of providing
extra fuel to cool (by
evaporation) the
engine.
 Carburetor tuning
must be done to
provide max power
without overheating
the 2 stroke engine.

3. VM Carburetor
Slide carburetor

provides smoother
variation and better
adjustment than a
throttle plate
carburetor.
VM is a Mikuni round

slide carburetor.
Various circuits control

fuel and air flow for
different conditions
◦ Starting, (idle) pilot, and
main

4. Integrated carburetor systems
The carburetor has

multiple adjustments
for different throttle
positions;
◦ 0-1/4 Pilot jet
◦ 1/8-1/2 Throttle cutaway
◦ ¼-3/4 Jet needle (clip)
◦ ¼-3/4 Needle jet
◦ ½-Full Main jet
◦ ½-Full Air adjustment
screw
◦ ½-full Power jet
(optional)

5. Starter Circuit
Mini- slide carburetor

used for starting
 “Choking” the
carburetor actually
opens plunger to
allow gas flow
 Gas is drawn from the
bottom of the bowl
through a starter jet
 Air is drawn from in
front of the slide

6. Starter circuit

7. Starter Circuit
Starter can only function when slide is

closed and vacuum is high
 Starter jet may need to be changed if
using a snowmobile carb on a
motorcycle, or vice-versa
 Plunger positions – Open
(choked), half-open, closed
 Starter circuit can be used to enrichen
mixture at other times to check
carburetor problems.

8. Pilot circuit
Used at idle to ¼

throttle
Gas flow is controlled

by pilot jet size, this jet
is the first to plug if gas
thickens
Air flow is controlled by

air adjustment needle
At idle, air flows in at 5

to further mix with fuel
At just off idle, mixed

fuel-air flows out at both

9. Pilot Circuit
Stumble coming off

idle indicates plugged
pilot jet or air screw
too tight
 Idle speed controlled
by a separate screw
that determines how
tightly the slide
closes.
 Slides with different
shape cutouts are
also sold for tuning
applications.

10. Mid range operation
¼ to ¾ operation is

controlled by needle
jet and jet needle. As
slide pulls up the long
tapered needle is
drawn out of needle
jet.
 The air jet controls
the air bled in to
atomize the fuel.
 The main jet is so
large that it does not
limit flow

11. Mid range operation
Both needle jet and

jet needles can be
changed, as can the
position of the clip on
the end of the needle.
 Needles can have up
to four different tapers
on them for different
performance
throughout the slide
travel.

12. Tuning
Simple tuning can

begin with moving the
needle up or down
one clip position.
This can be done
before changing
needles or needle
jets.
 Any tuning
adjustments should
be made by stepping
jets up or down one
size at a time.

13. Full throttle operation
At ¾ to full throttle, the

needle is so far
removed from the
needle jet, that it no
longer limits flow.
At this throttle

range, the main jet
limits gas flow.
Air jet limits air flow.

Always jet main rich

and work down.
Air jet is typically not


14. Power jets
Not found on all carbs – Provides richer mix at

top end

15. Jetting adjustments
To make proper

carburetor
adjustments the
mixture ratio at each
range will have to be
determined. There
are various methods:
Plug reading – plug

chop
Piston wash

Pyrometer readings

Condition corrections


16. Plug Reading – Plug Chop
To get an accurate read

of the condition, perform
a “plug chop”
◦ Install a new plug
◦ Warm engine up
◦ Run engine at selected
throttle setting for 30
seconds – 1 minute.
◦ Cut ignition and
simultaneously close-off
carb
◦ Study insulator down
toward the base of the
insulator, not on the tip!
◦ Appearance of tip better for
determining detonation

17. Plug appearance

18. Center electrodes
Another valuable indicator on

the spark plug is the center
firing electrode; as the plug
color starts to lighten up when
properly jetted, the center
firing electrode will start to
have a “silver” tip, or crown. As
the mixture becomes
leaner, this silver crown will
start to creep down the side of
the electrode; this is your
target. This metallic
appearance on the end of the
electrode should not extend
any further than 1/4 - 1/3 of the
way down the tip. Many tuners
are happy with the margin
afforded by simply seeing the
silver tip; then they’re close
enough for trail riding with a bit
of margin to spare.

19. Side electrode
The ground electrode is

also an indicator; on
many engines you will
see a “shadow” (darker
area) just up to the radius
(bend) that will usually
coincide with the color
and firing electrode
appearance. If this
shadow is further down
the ground strap towards
the plug threads, you’re
likely too lean. If this dark
shadow is all the way
across the strap to the
center of the plug, you’re
too rich.

20. NGK plug numbering system
The plug heat range can be

adjusted to keep a plug tip
clean, or avoid detonating on
an engine that otherwise has
the right mixture.
For NGK and most foreign

manufacturer’s spark plugs
there is a simple rule of
thumb:Low heat rating number
(for inst. BP4ES) quot;Hot spark
plugquot;.high heat intake due to
long insulator tip.
.High heat rating number (for

inst. BP8ES) quot;Cold spark
plugquot;.Low heat intake, due to
short insulator tip.
For Champion and most

domestic manufacturer’s spark
plugs (e.g. in ATVs, the
numbering is opposite.

21. Piston reading
Normally done for an engine that has been run for ¼

mile at constant speed. By looking down the spark
plug hole with piston at BDC, a clear look at the top
of the piston can determine how much carbon is left.
A ½” ring of wash (clean) is “normal”, typically a
little variation at transfer and exhaust ports will be
noted.

22. Exhaust gas pyrometers
Uses the Exhaust

temperature as a check
on mixture, and relative
changes, not for baseline
tuning. Thermocouples
are typically installed
permanently 5-10” from
exhaust port and they
must be centered in pipe.
Manufacturers will often
give you an exact
location.

23. Interpreting EGT values
Each gauge is used

for relative readings
◦ Guidelines
 Below 1100 – Rich
 1200-Ideal
 Above 1250 Lean
Changing a Mikuni
carburetor jet one size
will cause about a 50
degree temperature
change

24. Relative Air Density correction
Once an ideal jetting has

been developed for one
condition, it will need to
be corrected for changing
environmental conditions:
◦ Temperature (Loss of 2%
per 10 degree F increase)
◦ Altitude (loss of 3.5% per
1000’ increase)
◦ Barometric pressure
◦ Relative Humidity – Moist
air is less dense

25. Relative Air density
calculations
Calculating Air Density

Air density is expressed in units of mass per unit volume, in this

case kg/m3. The formula for this calculation is derived from basic
physics.
Air Density (kg/m3) = 1.2929 * (273.13/(T+273.13)) * ((P-

MN*RH)/760)
Simplified = 0.46464*(P-MN*RH)/(T+273.13)

where

T = temperature in Celsius

P = barometric pressure in mm of mercury

MN = moisture number from the following chart - include chart

RH = relative humidity, expressed as a decimal (0.55, not 55%)

For example:

T = 13C

P = 770 mmHg

MN per the chart for 13C is 11.24

RH = .21 (21%)

Air Density = 1.25 kg/m3


26. Relative air density use
In simple form, the change in relative air density

corresponds to the change in jet size.
◦ e.g. – If relative air density goes from 100 to 103 (a 3
% change) the jet size should increase by 3%. If you
are using a 300 jet, the new jet size will be (300 x
1.03) = 309. Since jets only come in multiples of
5, you could use a 310.
 Jetting changes in direct proportion to changes in absolute
temp on the Rankine scale (460 + Farenheit temp)
 Jetting changes only 70% as much as absolute barometric
pressure changes. This is due to the fact that the bowl is
vented, and less pressure is exerted on the gas in the bowl as
well
 Hex head jets are marked in area, round head jets in diameter.

27. Correcting jet size for RAD
For instance, if you get the jetting perfect at AD = 1.05

and the air density changes to 1.08, you know that you
will need to go up at least one size. If it goes from 1.05
to 1.06, is it time to change a jet or not? This is why you
need to check all the way through the range. I know that
for my bike, AD = 1.03-1.05 is one jet size and 1.06-1.08
is the next size up. In the above example, I would have
known to go up one size. If it had gone up to 1.08 and
looked like it was going to continue rising, I would go up
two sizes.

28. Engine sound
An engine running

way too rich will “four
cycle” or miss ignition
on every other stroke.
 An engine running
way too lean will be
running very fast and
not return to idle

29. TM Carburetor
TM is a flat slide carb

 Less disruption of air
flow in venturi gives
carbs better response
and power.
 Rack mounted for
twins and triples
reduce throttle
effort, and are held in
synchronization
 Circuits similar to VM

30. Mikuni CV Carb
Used on four

strokes, this is a
“Constant Velocity” or
“Constant Vacuum”
carb.
 Butterfly throttle is
combined with a slide
to give the carb good
response to quick
throttle opening.
 Butterfly controlled by
operator, Slide is
controlled by vacuum

31. CV Carb Operation
Operator controls

butterfly from idle to ¼
throttle, during this slide
is held ¼ open
Past ¼ throttle, the

engine vacuum is applied
to the slide, and the slide
movement is controlled
by vacuum
Slide will not open until

sufficient engine vacuum
has been developed.
Opening throttle quickly

will not produce “flat spot”
Setting Idle speed too

high will cause carburetor
to “hang”, or not return to
idle

32. Carburetor size
Carburetor Throttle

Carburetor size must

Bore Diameter
be matched to flow
D = K x SQRT( C x N )

for displacement and
D is throttle bore

rpms that engine is
diameter, in millimeters
used. Changing
K is a constant (

either will require an approx. 0.65 to 0.9,
adjustment to the derive from existing
carburetor to match. carburetor bore)
C is cylinder

displacement, in liters
N is RPM at peak

power

33. Tuned intake
Resonance Effects

Intake (airbox) can

also be “tuned” to F = Vs / 2¼ * the square

root of A / Vc (L + 1/2 the
match resonance of
square root of ¼ A
intake with peak
Vs is the sonic speed

power band. Usually about 1100 ft/sec)
A is the cross-sectional

area of the inlet
L is the inlet pipe length

Vc is the flask (crankcase)

volume

34. Ignition safety
To avoid having the

slides freeze open and
customer stuck on a
runaway sled, the safety
switch on the side of carb
measures a magnet on
the slide to be sure the
slide closes when throttle
is released.
When replacing throttle

cable, BE SURE TO SET
FREE PLAY or engine
may not start