Polaris 340 LX 2007, Service Manual

Page: 89 / 327

4.9
FUEL DELIVERY
4
Fuel Screw
The fuel screw controls the fuel mixture from idle to low speeds.
The tapered tip of the fuel screw projects into the passage
leading to the by pass out let. By turning the screw in or out, the
cross sectional area of the air passage is varied, in turn varying
the fuel screw varies the amount of air/fuel.
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.
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.
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 Owner’s Manual
Supplements.
intake side