Fuel Pump Description/Operation; Checking For Restricted Fuel Flow Caused By Anti-Siphon Valves - Mercury 225 EFI Service Manual

FUEL PUMP

Fuel Pump Description/Operation

The fuel pump is a crankcase-pressure-operated, diaphragm-type pump. Crankcase pul-
sating pressure (created by the up-and-down movement of piston) is transferred to fuel
pump by way of a passage (hole) between crankcase and fuel pump.
When piston is in an upward motion, a vacuum is created in the crankcase, thus pulling
in on the fuel pump diaphragm. The inlet check valve (in fuel pump) is opened and fuel
(from fuel tank) is drawn into fuel pump.
Downward motion of the piston forces the air mixture out of the crankcase into the cylin-
der. This 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 VST (EFI models) or carburetors.
Checking for Restricted Fuel Flow Caused by Anti-Siphon
Valves
While anti-siphon valves may be helpful from a safety stand-point, they clog with debris,
they may be too small, or they may have too heavy a spring. Summarizing, the pressure
drop across these valves can, and often does, create operational problems and/or power-
head damage by restricting fuel to the fuel pump and VST. Some symptoms of restricted
(lean) fuel flow, which could be caused by use of an anti-siphon valve, are:
1 - Loss of fuel pump pressure
2 - Loss of power
3 - High speed surging
4 - Preignition/detonation (piston dome erosion)
5 - Outboard cuts out or hesitates upon acceleration
6 - Outboard runs rough
7 - Outboard quits and cannot be restarted
8 - Outboard will not start
9 - Vapor lock
Since any type of anti-siphon device must be located between the outboard fuel inlet and
fuel tank outlet, a simple method of checking [if such a device (or bad fuel) is a problem
source] is to operate the outboard with a separate fuel supply which is known to be good,
such as a remote fuel tank.
If after using a separate fuel supply it is found that the anti-siphon valve is the cause of
the problem, there are 2 solutions to the problem: 1) replace the anti-siphon valve with
one that has lighter spring tension or 2) replace it with a solenoid-operated fuel shut off
valve.
Page 3A-6 90-884294 OCTOBER 2001