Polaris 2012 Sportsman Forest 500 International, Service Manual

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4.11
CARBURATION AND FUEL SYSTEM
© Copyright 2011 Polaris Sales Inc.
9923412 - 2012 Sportsman 400/500 and EFI Tractor Service Manual
Air/Fuel Mixture Ratio
A carburetor with a slide type throttle valve is also called
a variable venturi type carburetor. In this type of
carburetor, the needle jet and jet needle serve to control
proper air/fuel mixture ratio at the medium throttle valve
opening (between 1/4 and 3/4 opening).
Having the proper needle jet and jet needle has a major
impact on engine performance at partial load. 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” clip inserted into one of
the five slots provided in the head of the jet needle. The
previous chart shows the variation of fuel flow based on
the height of the “E” clip.
Jet Needle
The jet needle has five adjustment grooves cut into the
upper portion, and is tapered from approximately the
middle of the needle to the lower end. The top is fixed to
the center of the throttle valve by the needle clip, and the
tapered end extends into the needle jet. Fuel flows through
the space between the needle jet and jet needle. This
space does not vary until the throttle reaches the 1/4 open
point. At that time the tapered portion of the needle begins
to move out of the jet, affecting fuel flow as the opening
enlarges. If the needle clip is changed from the standard
position to a lower groove, the needle taper starts coming
out of the jet sooner, resulting in a richer mixture. Moving
the clip higher produces a leaner mixture. If the taper is
worn due to vibration, fuel flow may be significantly
affected.
Needle Jet
The needle jet works in conjunction with the jet needle to
regulate fuel flow rate. An air bleed opening in the side of
the needle jet brings in air measured by the air jet. This air
initiates the mixing and atomizing process inside the
needle jet.
E-Clip Position vs. Fuel Flow
Throttle Valve Opening
2
3
4
4
3
2
15
50
75
100%
Leaner
Richer
Position#
Jet Needle
Needle Jet
Main Jet