Snowmobile Polaris (2006 year). Manual - part 24

4.6

CARBURETION

MIKUNI TM 38

CARBURETOR OVERVIEW

The advantages of the TM flatslide system include improved 
throttle response and a significant reduction in throttle effort 
due to rack style carbs and the use of cable cam. The 
following are the main components of TM carburetors and the 
functions of each components.

Pilot Air Jet (A): The pilot air jet controls the amount of air 
entering the engine in the idle circuit. The pilot air jet size is 
imprinted on it. Bigger pilot air jets allow more air to enter, 
leaning the idle mixture. If engine loads up at idle, first try 
turning fuel screw in. If not correct, try larger pilot air jet.

Starter Jet (B): The starter jet's function is to meter fuel 
entering the engine when choked. Larger numbered starter jets 
pass more fuel, therefore more fuel will enter the system when 
jetting to a larger size. Like the pilot air jet, the jet size is 
imprinted.

Jet Needle (C): The jet needle performs the same functions as 
on Mikuni VM carburetors. However, the needles in the TM-
38 carburetors are longer and are not interchangeable with 
VM needles. To raise or lower the needle, remove the top cap 
(D) and loosen the 2.5mm Allen screw holding the needle in 

place. Reach inside with a long nose pliers and pull the needle 
out. The C-clip can then be adjusted for the desired effect.

Needle Jet (D): The needle jet is press fit into carbs and is not 
replaceable.

Piston Valve or Throttle Valve): The throttle valve controls 
the rate of engine air intake by moving up and down inside the 
main bore. At small throttle openings, air flow control is 
performed chiefly by the cutaway. By controlling air flow the 
negative pressure over the needle valve is regulated, in turn 
varying the fuel flow

Main Jet and Pilot Jet (F): The main jets (hex) and pilot jets 
(air bleed type) are identical in style and function as the 
Mikuni VM carbs. There is washer on main jet that is staked 
and should not come out. The part numbers for main jets and 
pilot jets are the same as Mikuni VM round slide carburetors.

Fuel Screw (G): The fuel screw controls the low speed air/
fuel mixture at idle. Turning the fuel screw out makes the 
mixture richer and can be adjusted up to five turns out.

Float/Needle & Seat Assembly (H): The float/needle & seat 
are sold as an assembly and cannot be replaced as individual 

4.7

CARBURETION

components. The float level is pre-set in the molding and 
cannot be adjusted. 

Carburetor Synchronization: Remove the top caps on all 
carburetors. In the body of the carburetors, there is a Phillips 
head set screw (I) that connects the slide lever to the throttle 
lever shaft. The carburetor with the fixed set screw is the base 
carburetor (middle carburetor on triples, PTO carburetor on 
twins) and the remaining carbs are synchronized to it. 
Carburetor synchronization is measured at wide open throttle. 
Open the throttle to wide open. The bottom of the slide should 
be flush with the top of the throttle bore. If it is not flush, 
locate the wide open throttle stop screw (J) and turn it until the 
base carburetor is set flush. Loosen the Phillips head set screw 
in the remaining carburetor(s). Turn the adjusting nut (K) that 
surrounds the set screw until the throttle slide is set the same 
as the base carburetor. Tighten all set screws and replace the 
top caps making sure gaskets are properly positioned.

MIKUNI CARBURETOR

FUNCTION

The function of a carburetor is to produce a combustible air/
fuel mixture by breaking fuel into tiny particles in the form of 

vapor, to mix the fuel with air in a proper ratio, and to deliver 
the mixture to the engine. A proper ratio means an ideal air/
fuel mixture which can burn without leaving an excess of fuel 
or air. Whether the proper mixture ratio is maintained or not is 
the key to efficient engine operation.

The engine of a vehicle is operated under a wide range of 
conditions, from idling with the throttle valve remaining 
almost closed, to full load or 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, or pilot system, and a main fuel system are 
provided in Mikuni type carburetors. The Mikuni carburetor 
has varying operations depending upon varying driving 
conditions. It is constructed of a float system, pilot system, 
main system, and starter system or initial starting device.

FLOAT SYSTEM

The float system is designed to maintain a constant height of 
gasoline during operation. When the fuel flowing from the fuel 
pump into the float chamber through the needle valve reaches 
the constant fuel level, the floats rise. When the buoyancy of 
the float and the fuel pressure of the fuel pump balance, the 
needle valve sticks fast to the needle seat, preventing further 
delivery of gasoline, thereby holding the standard level of 
gasoline.

The fuel level in the bowl assists in controlling 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, since not enough fuel leaves 
the nozzle. Therefore, the predetermined fuel level should not 
be changed arbitrarily.

WARNING

 DO NOT pressurize the fuel system by forcing compressed 
air through the fuel tank. Forcing air through the needle and 
seat will cause damage and the float/needle & seat assembly 
will have to be replaced.

4.8

CARBURETION

FUEL METERING

Mikuni carburetors use a starter enricher system rather than a 
choke. In this type of carburetor, 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 through 
the fuel discharge nozzle in the optimum air/fuel ratio. The 
starter is opened and closed by means of the starter plunger. 
The starter type carburetor is constructed to utilize the 
negative pressure of the inlet pipe, so it is important that the 
throttle valve is closed when starting the engine

FUEL 
DELIVERY

The pilot system's 
main function is to 
meter fuel at idle 
and low speed 
driving. Though 
its main function 
is to supply fuel at 
low speed, it does 
feed fuel 
continuously 
throughout the 
entire operating 
range.

Fuel for the pilot 
jet is drawn from 
the float bowl, mixed with air regulated by the air screw, and 
delivered to the engine through the pilot outlet.

The mixture is regulated to some degree by adjusting the air 
screw. When the air screw is closed, the fuel mixture is made 
richer as the amount of air is reduced. When the air screw is 
opened, the mixture is made more lean as the amount of air is 
increased.

The main system is designed to deliver fuel between low 
speed and high speed operation. This system is made up of the 
jet needle, needle jet, and main jet. The main system begins to 
take effect as soon as there is enough air flow into the 
carburetor venturi to draw fuel up through the main jet and 
needle jet assembly. This system works in conjunction with 
the needle jet system.

During low speed driving, there is very little clearance 
between the jet needle and the needle jet; therefore, very little 
fuel from the main jet can pass between the jet needle and the 
needle jet. As the throttle valve opening is increased, the 
tapered jet needle is raised farther out of the needle jet, 
allowing greater fuel flow. Under full throttle opening, the 
cross sectioned area of clearance between the jet needle and 
the needle jet becomes greater than the cross sectioned area of 
the main jet. Thus the main jet is now controlling the amount 
of fuel flow.

PILOT JET

From idling to low speeds, the fuel supply is metered by the 
pilot jet. There are several air bleed openings in the sides of 
the pilot jet which reduce the fuel to mist. The number 
stamped on the jet is an indication of the amount of fuel in cc's 
which passes through the jet during a one minute interval 
under a given set of conditions.

PILOT AIR SCREW

The pilot air screw controls the fuel mixture from idle to low 
speeds. The tapered tip of the air screw projects into the air 
passage leading to the pilot jet air bleeds. By turning the screw 
in or out, the cross sectional area of the air passage is varied, 
in turn varying the pilot jet air supply and changing the 
mixture ratio.

MAIN JET

When the throttle 
opening becomes 
greater and the area 
between the needle jet 
and jet needle 
increases, fuel flow is 
metered by the main 
jet. The number on the jet indicates the amount of fuel cc's 

FUEL METERING

4.9

CARBURETION

which will pass through it in one minute under controlled 
conditions. Larger numbers give a greater flow, resulting in a 
richer mixture. Main jets are screwed directly into the needle 
jet base.

JETTING GUIDELINES

Changes in altitude and temperature affect air density, which is 
essentially the amount of oxygen available for combustion. In 
low elevations and cold temperatures, the air has more oxygen. 
In higher elevations and higher temperatures, the air is less 
dense.

Verify the production setting for your specific model. All 
carburetors must be re-calibrated if operated outside the 
production temperature and/or altitude range. The main jet 
installed in production is not correct for all altitudes and/or 
temperatures. Refer to the jetting cart in the Specifications 
Chapter of this manual for correct jetting for altitude/
temperature ranges.

NOTE:  It is the owner's responsibility to ensure that 
the correct jets are installed in the machine for a 
geographical area. Be very careful when jetting 
down in warm weather. As the weather turns colder 
it will be necessary to re-jet upward to prevent 
engine damage. When selecting the proper main jet 
always use the lowest elevation and temperature 
that is likely to be encountered.

PISTON VALVE or THROTTLE VALVE

The throttle valve controls the rate of engine air intake by 
moving up and down inside the main bore. At small throttle 
openings, air flow control is performed chiefly by the cutaway. 
By controlling air flow the negative pressure over the needle 
valve is regulated, in turn varying the fuel flow.

The throttle valves are numbered 1.0, 1.5, 2.0, etc., according 
to the size of the cutaway in millimeters. The higher the 
number, the leaner the gasoline/air mixture.

JET NEEDLE / NEEDLE JET

The jet needle and needle jet have the most effect between 3/8 
and 3/4 throttle opening. Some mixture adjustment can be 
accomplished by changing the location of the E" clip on the 
needle. Moving the clip down raises the needle in the jet 
passage and richens the mixture. Moving the clip up lowers 
the needle in the jet passage and leans the mixture. Letter and 
number codes are stamped into the needle and the jet 
indicating sizes and tapers (taper of needles only) of each.

JET NEEDLE

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 the “E" ring inserted into one of the five slots 

CAUTION

A Main Jet that is too small will cause a lean operation 

condition and may cause serious engine damage. jet 

the carburetors carefully for elevation and temperature 

according to the jetting charts in the Specifications 

Chapter of this manual or the models Owners Manual 

Supplements

intake side