2003 Polaris Deep Snow Snowmobiles Service Manual, Page 157Get this manual
FUEL SYSTEMCARBURETION The fuel pumps on all Polaris engines are basically the sameThe differences are in the size and location of the pumpsPumps may be mounted to the engine or to the chassisIn the two cycle engine, the pressure in the crankcase changes with the up and down stroke of the pistonThe amplitudes of pressure vary according to the RPM and degree of throttle openingWhether idling or at full throttle, the pressure built up in the crankcase has enough amplitude to operate the pumpWhen the piston is on the upstroke, crankcase pressure in that cylinder becomes less positiveThe diaphragm in the fuel pump moves toward the engine, causing negative pressure or suction in the pump chamberThis causes the inlet valve from the fuel supply to open and permits fuel to enter the chamberThis same suction causes the outlet valve (to the carburetor) to close so that fuel cannot return form the carburetorWhen the piston begins its downward stroke, the pressure from the crankcase becomes positive, causing the fuel pump diaphragm to move in the opposite direction and reversing the pressure in the fuel pump chamberThis causes the inlet valve in the pump to close and the outlet valve to open, filling the float bowl in the carburetorWhen the float level in the carburetor reaches its standard level, the needle valve will close, preventing more fuel from entering the carburetor, even though the fuel pump continues to try to provide the carburetor with fuel