Mercury Mariner Outboard 225 3 Litre Service Manual 1994, Page 111Get this manual
Fuel Pump General Information
FUEL PUMP DESCRIPTIONOPERATION
The fuel pump is crankcase-pressure-operateddia-
CHECKING FOR RESTRICTED FUEL FLOW CAUSED BY ANTI-SIPHON VALVES While anti-siphon valves may be helpful from safety stand-pointthey clog with debris, they may be too small or they may have too heavy springThe pressure drop across these valves can create operational problems andl or powerhead damage by restricting fuel flow to the fuel pump and carburetor(s) Some symptoms of restricted (lean) fuel flow which could be caused by use of an anti-siphon valve are: 1Loss of fuel pump pressure
phragm-type
pumpCrankcase
pulsating
pressure
(created by the up-and-down movement of piston) is trans-
ferred to fuel pump by way of passage (hole) between crankcase and fuel pump
When piston is in an upward motion, vacuum is created in the crankcase, thus pulling in fuelair mixture (from carbu retor) into crankcaseThis vacuum also pulls in on the fuel pump diaphragm, thus the inlet check valve (in fuel