1972 Ski-Doo Shop Manual, Page 171Get this manual
CARBURETOR of the engineThe action of the metering diaphragm is the same as previously described with suction required to operate the diaphragm being transmitted through the main fuel discharge port-
Duplex Pump: Used on large carburetors having higher fuel consumption
NOTE: When disassembling carburetor, as certain whether it is single or duplex pumpMisplacement of diaphragms can lead to carburetor malfunction
FUEL PUMP OPERATION The fuel pump is pulse operated diaphragm pumpThe pressure-vacuum pulse is supplied from the engine crankcase where the pulse cycles are created by the reciprocating action of the engine pistonCrankcase pulse is transmitted to the pump pulse chamber through the fuel pump pulse port in the mounting flange of the carburetor bodyVacuum Action: The vacuum part of the pulse cycle causes the fuel pump diaphragm to move into the pump pulse chamberThe vacuum allows fuel to flow from the fuel filter through the fuel strainer screen, past the inlet check valve and into the fuel pump chamberThe outlet check valve closes during this part of the pumping cyclePressure Action: The pressure part of the pulse cycle forces the fuel pump diaphragm into the fuel pump chamber, creating pressure that forces the fuel out through the outlet check valve and the fuel inlet supply channel to the inlet needle valveThe fuel pressure closes the inlet check valve during this part of the pumping cycle