Theory Of Operation; Troubleshooting - Mercury 40 Service Manual

Theory of Operation

The fuel pump is a crankcase-pressure-operated, di-
aphragm-type pump. Crankcase pulsating pressure
is transferred by way of a passage (hole) from the
crankcase to the fuel pump.
When the piston travels upward, a vacuum is created
in the crankcase. This vacuum pulls in the fuel pump
diaphragm, the inlet check valve (in fuel pump) is
opened, and fuel is drawn into fuel pump.
Downward motion of the piston forces out the fuel
pump diaphragm, closes the inlet check valve (to
keep fuel from returning to fuel tank) and opens the
outlet check valve, forcing fuel to the carburetors.

Troubleshooting

WARNING
FIRE AND EXPLOSION HAZARD. Observe fire
prevention rules, particularly NO SMOKING. Be-
fore servicing any part of the fuel system, discon-
nect electrical system at the battery. Drain the
fuel system completely. Use an approved con-
tainer to collect and store fuel. Wipe up any spil-
lage immediately. Materials used to contain spil-
lage must be disposed of in an approved
receptacle. Any fuel system service must be per-
formed in a well ventilated area
FUEL LEAKAGE FROM ANY PART OF THE FUEL
SYSTEM CAN BE A FIRE AND EXPLOSION HAZ-
ARD WHICH CAN CAUSE SERIOUS BODILY IN-
JURY OR DEATH. Careful periodic inspection of
the entire fuel system is mandatory, particularly
after engine storage. All fuel components, in-
cluding fuel tanks, whether plastic, metal, fiber-
glass, fuel lines, primer bulbs, and fittings, must
be inspected for corrosion. Any sign of leakage
or deterioration necessitates replacement before
further engine operation.
3A-8 - FUEL SYSTEM
.
Checking For Restricted Fuel Flow
Caused By Anti-Siphon Valves
While anti-siphon valves are helpful from a safety
stand-point, they clog, they may be too small, or they
may have too heavy a spring. The pressure drop that
occurs with these valves can create operational
problems and/or powerhead damage by restricting
flow of fuel. Some symptoms of restricted (lean) fuel
flow, are:
 Loss of fuel pump pressure
 Loss of power
 High speed surging
 Preignition/detonation (piston dome erosion)
 Outboard hesitates upon acceleration
 Outboard runs rough
 Outboard quits and cannot be restarted
 Outboard will not start
 Vapor lock
Any type of anti-siphon device must be located be-
tween the outboard fuel inlet and fuel tank outlet. A
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.
If 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 a solenoid-operated fuel shutoff
valve.
90-852572R1 JANUARY 1998