Starting / Enrichment System; Float System; Idle Speed Adjustment; Air Screw - Polaris 340 LX 2007 Service Manual

Starting / Enrichment System

The starting or enrichment system provides additional fuel
required to start the engine in cold conditions. A cold
engine requires more fuel to start than a warm engine
because the fuel mixture tends to condense on the intake
and cylinder walls rather than vaporize as it would in a
engine at operating temperature.
Mikuni VM carburetors use a plunger-type starting
system. When the choke lever is engaged, a spring-
loaded plunger is pulled away from the starting circuit
orifice allowing additional fuel to be drawn out of the float
bowl along with the fuel drawn through the pilot jet system.
Because the starting system provides fuel not metered by
the pilot or main jet systems, it is important to verify the
plunger properly closes when not needed. A starting
system that is stuck open or improperly set will cause rich
running and heavy plug fouling.
Likewise, a plunger that is stuck closed or does not open
far enough will make starting a cold engine difficult.

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
predetermined fuel level should not be changed arbitrarily.

Idle Speed Adjustment

The idle speed adjustment screw sets the height of the
throttle slide inside of the mixing chamber. Turning the
screw in (clockwise) raises the throttle slide and raises
engine RPM. Turning the idle speed screw out (counter-
clockwise) lowers the throttle slide and lowers engine
RPM.
Always set the idle speed after the engine reaches
operating temperature.
nozzle. Therefore, the
9923396 - 2007-2012 EDGE/Widetrak LX Service Manual
©2011 Polaris Sales Inc.

Air Screw

The air screw controls the amount of air that is mixed with
the fuel drawn out of the float bowl. The more air allowed
to mix with the fuel, the leaner the fuel mixture. Turning the
screw in (clockwise) reduces the amount of air, while
turning the screw out (counter-clockwise) increases the
amount of air.

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.

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
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. At higher elevations and in 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 to prevent engine damage. When
Fuel System
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4.5