1987-1993 Mercury Mariner Outboards 70/75/80/90/100/115HP 3 and 4-cylinder Factory Service Manual, Page 88Get this manual
Fuel Pumps General Information
FUEL PUMP DESCRIPTIONOPERATION
If, after using separate fuel supply, it is found that the anti-siphon valve is the cause of the problem, there are solutions to the problem; either (1) remove the anti-siphon valve or (2) replace it with solenoidoperated fuel shutoff valve
The fuel pump is crankcase-pressure-operated, diaphragm-type pumpCrankcase pulsating pressure (created by the up-and-down movement of piston) is transferred to fuel pump by way of passage (hole) between crankcase and fuel pumpWhen piston is in an upward motion, vacuum is created in the crankcase, thus pulling in fuelair mixture (from carburetor) into crankcaseThis vacuum also pulls in on the fuel pump diaphragm, thus the inlet check valve (in fuel pump) is opened and fuel (from fuel tank) is drawn into fuel pumpDownward motion of the piston forces the fuelair mixture out of the crankcase into the cylinderThis motion also forces out on the fuel pump diaphragm, which, in turn, closes the inlet check valve (to keep fuel from returning to fuel tank) and opens the outlet check valve, thus forcing fuel to the carburetorsCHECKING FOR RESTRICTED FUEL FLOW CAUSED BY ANTI-SIPHON VALVES