Polaris Trail Blazer, Service Manual

Page: 284 / 665

4.19 Polaris Sales Inc. 4/99
FUEL SYSTEM/CARBURETION
Mikuni CV Carb Operation
The constant velocity carburetor used on Polaris 4 Cycle ATVs and 6x6 incorporates a mechanically operated
throttle plate and a vacuum controlled slide valve (vacuum slide). The venturi cross-sectional area in the carbure-
tor 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. When
the pressure above and below the diaphragm are nearly equal, the slide moves downward under spring pressure.
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 34 / BST 40 carburetor.
Air Flow
Low Pressure
Air Flow
Low Pressure
Air Box
Pressure
From
Air Box
Venturi
Low Pressure
From Venturi
From
Air Box
Throttle
Plate
Throttle
Plate
Diaphragm
Diaphragm
Vacuum
Slide
Vacuum
Slide
Low Pressure
From Venturi