Polaris SPORTSMAN 400 2003, Service Manual

Page: 112 / 274

FUEL SYSTEM/CARBURETION
4.7
CARBURETOR OPERATION CONT’D
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.
= Air Flow = Low Pressure
Low Pressure
From Venturi
From
Air Box
Throttle
Plate
Diaphragm
Vacuum
Slide
Venturi
Note: Diagrams are for explanation of theory only, and are not true
representations of Mikuni BST carburetor.
PILOT (IDLE AND SLOW)
SYSTEM
This system supplies fuel during engine operation
with throttle valve closed (1) or slightly opened. The
fuel from float chamber (2) is metered by pilot jet (3)
where it mixes with air coming in through pilot air jet
(4). The mixture then goes up through pilot passage
to pilot screw (5). A part of the mixture is discharged
into the main bore out of bypass ports (6).
The
remainder is then metered by pilot screw and
discharged into the main bore through pilot outlet (7).
4
1
6
7
5
2
3
STARTER SYSTEM (CHOKE
OR ENRICHMENT)
When the choke cable (1) is activated, the starter
plunger (5) is lifted off the seat.
1
5
8
6
4
2
3
7
Fuel is drawn into the starter circuit from the float
chamber (2) through the starter jet (3). Starter jet
meters this fuel, which then flows into starter pipe (4)
and mixes with the air (7) coming from the float
chamber. The mixture, rich in fuel content, reaches
starter plunger and mixes again with the air coming
through a passage (8) extending from underneath the
diaphragm. The rich fuel/air mixture for starting is
discharged through starter outlet (6) in the the main
bore.