1997+ Mercury 35/40HP 2 Cylinder Outboards Service Manual PN 90-826148R2, Page 139Get this manual
Theory of Operation The fuel pump is crankcase-pressure-operated, diaphragm-type pumpCrankcase pulsating pressure is transferred by way of passage (hole) from the crankcase to the fuel pumpWhen the piston travels upward, vacuum is cre ated in the crankcaseThis vacuum pulls in the fuel pump diaphragm, the inlet check valve (in fuel pump) is opened and, and fuel is drawn into fuel pumpDownward motion of the piston forces out the fuel pump diaphragm, closes the inlet check valve (to ke ep fuel from returning to fuel tank) and opens the outlet check valve, forcing fuel to the carburetors
Checking For Restricted Fuel Flow Caused By Anti-siphon Valves
While anti-siphon valves are helpful from safety stand-point, they clog, they may be too small, or they may have too he avy springThe pressure drop that occurs with these valves can cre ate operational problems andor powerhe ad damage by restricting flow of fuelSome symptoms of restricted (le an) fuel flow, are: Loss of fuel pump pressure Loss of power High spe ed surging Preignitiondetonation (piston dome erosion) utboard hesitates upon acceleration utboard runs rough utboard quits and cannot be restarted utboard will not start Vapor lock Any type of anti-siphon device must be located betwe en the outboard fuel inlet and fuel tank outletA method of checking [if such device (or bad fuel) is problem source] is to operate the outboard with separate fuel supply which is known to be goodIf it is found that the anti-siphon valve is the cause of the problem, either 1) replace the anti-siphon valve, or 2) replace it with solenoid-operated fuel shutoff valve