Fuel / Exhaust
4.6
Keihin Carburetor System Overview -- Continued
Pop Off Pressure
Carburetor pop-off pressure is pre-set at the factory for a given engine. Test as outlined on following page. If pop-off
pressure is incorrect, follow the procedure below to find the cause. Pop-off pressure should not be changed to correct
a running condition problem unless required by changes in elevation, or engine/carburetor modifications for racing.
Fuel inlet needle “pop off” pressure is influenced by many factors. Atmospheric pressure, venturi vacuum (low pres-
sure), the amount of spring pressure on the control arm, fuel pressure from the fuel pump, and the size of the needle
and seat all have an affect on operating pop off pressure. All of these forces combined regulate the amount of fuel
that enters the carburetor fuel chamber and the engine.
Needle Valve / Diaphragm Operation
The amount of fuel allowed into the fuel
chamber is controlled by the inlet needle
valve assembly. Opening and closing of
the valve is controlled by regulator dia-
phragm movement and spring pressure
under the control arm.
The regulator diaphragm is sealed to the
carburetor body forming two chambers.
One side is vented to atmospheric pres-
sure, the other side (fuel chamber) is con-
nected to the venturi via the jets and pas-
sages. Atmospheric pressure pushes on
one side of the diaphragm, applying pres-
sure on the control arm in the fuel chamber.
This reduces pressure on the inlet needle
and allows it to lift more easily off the seat.
When the engine is running, pressure in the
carburetor venturi (and therefore the fuel
chamber) is less than atmospheric. This in-
creases the effect of the atmospheric pres-
sure on the diaphragm and lever . When re-
turn spring pressure is overcome by these
forces, the needle lifts off of the seat or
“pops off”, allowing pressurized fuel to enter
the fuel chamber.
Pop off pressure cannot be tested with the engine running. However, a static pressure test can help identify prob-
lems within each individual carburetor.
The measurement indicates the amount of pressure required to force the fuel inlet needle off of the needle seat
without the engine running. Thoroughly inspect all system components whenever a rich or lean condition is en-
countered.
NOTE: If inlet needle does not seal properly , fuel may enter the engine and cause flooding when the engine is off.
Low pop off pressure may be caused by:
S Foreign material in the tip of the needle or needle seat
S Worn, broken, or damaged lever arm return spring
S Leaking needle seat O-Ring
S Corrosion on lever arm, lever arm pin, and/or return spring
S Incorrect lever arm return spring (Use only genuine Polaris parts)
S Fuel return line kinked or return restrictor orifice plugged (in carburetors or in return line)
1
Regulator
Diaphragm
Return
Spring
Fuel
Chamber
Fuel Inlet
(Fuel Pressure)
(V enturi
Low Pressure)
Inlet Needle
Inlet
Needle
Seat
Sealing
O-Ring
Vent-
Atmospheric
Pressure
Diaphragm
Cover
Control Arm
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