Snowmobile Polaris IQ (2007-2008 year). Manual - part 22
4.6
Fuel Systems
MIKUNI TM 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.
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.