Polaris Virage TX 2000 Service Manual page 78

FUEL SYSTEM/CARBURETION
System Operation - Keihin
The Keihin carburetor uses a floatless diaphragm type. Several separate but overlapping systems are used to
meter fuel and air properly. Refer to the illustrations on page 3.16 - 3.17.
1. Air Metering System
The metering of air is controlled by the throttle plate. The position of the throttle plate determines which fuel deliv
ery system or systems supply fuel from the fuel chamber to the engine.
2. Inlet Needle Valve System/Fuel Chamber
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 diaphragm 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 atmo
spheric pressure, the other side (fuel chamber) is exposed to the venturi (low pressure) through the fuel delivery
jets and passages. Atmospheric pressure pushes on one side of the diaphragm, applying pressure 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 atmo
spheric. This increases the effect of the atmospheric pressure on the diaphragm and lever and overcomes spring
pressure. When the lever return spring pressure is overcome by these forces, the needle lifts off of the seat or
"pops off", allowing pressurized fuel from the fuel pump to enter the fuel chamber. Fuel inlet needle "pop off" pres
sure is influenced by many factors. Atmospheric pressure, venturi low pressure, the amount of spring pressure
on the control arm, and fuel pressure from the fuel pump 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.
Changes to pop-off pressure are not recommended.
3. Idle System
At idle and low throttle settings, fuel mixture is controlled by the idle jet and idle mixture screw. At idle, the throttle
plate is open only far enough to uncover the idle port. The idle mixture screw controls fuel delivery to the idle
port on Keihin carbs. When the throttle plate is closed, fuel metered through the idle jet is mixed with incoming
air through the bypass portsXo atomize the air/fuel mixture. For this reason, the idle speed must be set accurately
before any adjustments are performed to the idle mixture screw on Keihin carbs. As throttle opening increases,
fuel from the idle jet is discharged through the bypass holes to ensure proper mixture. The low speed adjusting
screw can be used to fine tune the idle and low speed system. Turning the screw in leans the mixture; turning
the screw out richens the mixture
4. Mid-Range System
At mid-range throttle openings (approximately 1/4 through 1/2 throttle) fuel mixture is determined primarily by the
mid-range jet or transition jet. The idle jet, idle mixture screw, and main jet, have a minor effect on mid-range
mixture. Fuel is metered through the mid-range jet or transition jet and discharged through the mid-range port.
As throttle opening increases, the main jet has a greater effect and the mid-range or transition jet a lesser effect.
The mid-range system does not have an adjustable mixture screw on the Keihin carburetors.
5. High Speed System
The high speed system consists of the main jet This has an effect on mixture from 1/4 to full throttle, with the
greatest affect at 1 /2 to wide open throttle. Compensating for altitude is accomplished only by changing main jets.
6. Choke System
The purpose of the choke system is to supply an overly rich mixture for starting a cold engine. When the choke
plate is fully closed and throttle plate is open a strong vacuum is applied to the main nozzle, mid-range, bypass,
and idle ports, and a very rich fuel air mixture is discharged to the engine for starting.
Polaris Sales Inc. 3.15 3/00