Polaris 340 LX 2007, Service Manual

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4.8
FUEL DELIVERY
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.
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 jet,
and delivered to
the engine through the pilot outlet.
The mixture is regulated to some degree by adjusting the fuel
screw.
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.
FUEL METERING