Polaris 2012 Sportsman Forest 500 International, Service Manual

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4.7
CARBURATION AND FUEL SYSTEM
© Copyright 2011 Polaris Sales Inc.
9923412 - 2012 Sportsman 400/500 and EFI Tractor Service Manual
CV Carburetor System Function Mikuni CV Carb Operation
The constant velocity carburetor incorporates a
mechanically operated throttle plate and a vacuum
controlled slide valve (vacuum slide). The venturi cross–
sectional area in the carburetor bore is increased or
decreased automatically by the vacuum slide, which
moves according to the amount of negative pressure (less
than atmospheric) present in the venturi.
A diaphragm attached to the top of the vacuum slide is
sealed to the slide and to the carburetor body forming two
chambers. The chamber above the diaphragm is
connected to the venturi area by a drilled orifice in the
center of the vacuum slide. The chamber below the
diaphragm is vented to atmospheric pressure by a
passage on the air box side of the carburetor. A spring,
installed in the center of the vacuum slide, dampens the
slide movement and assists the return of the slide.
When the throttle plate is opened and engine speed
begins to increase, the pressure in the venturi (and
therefore in the chamber above the diaphragm) becomes
significantly lower than atmospheric. Atmospheric
pressure in the chamber below the diaphragm forces the
diaphragm upward, raising the slide against spring
pressure.
Carburetor Component Function
System
Main
Components
Main
Function
Main
Affect
Float
System
(Level
Control)
Inlet Pipe,
Needle and
Seat, Float,
Float Pin Maintains
specified fuel
level in float
chamber
(carburetor float
bowl) All systems

All throttle
ranges
Venting Passages in
Carburetor, Vent
lines

to frame
Supplies

atmospheric
pressure to float
chamber
All systems

All throttle
ranges
Starter

(Choke/
Enrichment)
Choke Lever,
Cable, Plunger,
Return Spring,
Carb Passages
(Starter Jet,
Starter Bleed
Pipe) Supplies
additional fuel
air mixture
necessary for
cold starting All throttle
ranges

Greatest
effect at low
throttle
settings and
idle
Pilot (Idle
System) Pilot Jet/
Passage-ways,
Pilot–Mixture
Screw with
Spring Washer
and Sealing O–
Ring, By-pass
Ports (Behind
Throttle Plate),
Pilot Air Jet, Pilot
Outlet, Throttle
Plate Primarily
supplies fuel at
idle and low
throttle positions Mainly idle
to 1/4
throttle

Minimal
effect after
1/2 throttle
Main
System Main Jet, Main
Air Jet, Main Air
Passage,
Needle Jet, Jet
Needle, Vacuum
Slide, Throttle
Plate Supplies fuel at
mid–range and
high throttle
settings. 1/4 to full
throttle
Throttle Plate
Diaphragm
Low Pressure
From Venturi
Atmospheric
Pressure
Air inlet
Atmospheric
Vacuum
Slide
Venturi
= Air Flow
= Low pressure