Polaris Sportsman 500 Service Manual

FUEL SYSTEM/CARBURETION
4.6
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 atmo-
spheric) 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 . Aspring, installed in the center of the vacuum slide, dampens the slide movement
and assists the return of the slide.
When the throttle plate is openedand enginespeedbeginsto increase,the pressureintheventuri (andtherefore
in the chamber above the diaphragm) becomes significantly lower than atmospheric. Atmospheric pressure in
thechamberbelowthediaphragmforces thediaphragmupward, raisingthe slideagainst springpressure. When
thepressure aboveandbelowthediaphragm arenearlyequal, theslide moves downwardunder springpressure.
Raising or lowering the slide increases or decreases the cross sectional area in the venturi, and therefore the air
velocity in the venturi is kept relatively constant. This provides improved fuel atomization and optimum fuel/air
ratio.
Note: Diagrams are for explanation of theory only, and are not true representations of Mikuni BST carburetor.
Air Flow
Low Pressure
Air Flow
Low Pressure
Air Box
Pressure
From
Air Box
Venturi
Low Pressure
From V enturi
From
Air Box
Throttle
Plate
Throttle
Plate
Diaphragm
Diaphragm
Vacuum
Slide
Vacuum
Slide
Low Pressure
From V enturi

Polaris Sportsman 500 Specifications

General

Brand	Polaris
Model	Sportsman 500
Category	Offroad Vehicle
Language	English