Fuel Pumps; General Information; Fuel Pump Description/Operation; Checking For Restricted Fuel Flow Caused By Anti-Siphon Valves - Mercury 45 Jet 50-55 Service Manual

WARNING
!
FIRE AND EXPLOSION HAZARD. Observe fire preven-
tion rules, particularly NO SMOKING. Before servicing
any part of the fuel system, disconnect electrical sys-
tem at the battery. Drain the fuel system completely.
Use an approved container to collect and store fuel.
Wipe up any spillage immediately. Materials used to
contain spillage 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 SYS-
TEM CAN BE A FIRE AND EXPLOSION HAZARD
WHICH CAN CAUSE SERIOUS BODILY INJURY OR
DEATH. Careful periodic inspection of the entire fuel
system is mandatory, particularly after engine storage.
All fuel components, including fuel tanks, whether
plastic, metal, or fiberglass, fuel lines, primer bulbs,
fittings, swelling, and must be inspected for corrosion.
Any sign of leakage or deterioration necessitates re-
placement before further engine operation.

Fuel Pumps

General Information

FUEL PUMP DESCRIPTION/OPERATION

The fuel pump is a crankcase-pressure-operated, dia-
phragm-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, and fuel
is drawn into fuel pump.
Downward motion of the piston forces out the fuel pump di-
aphragm, closes the inlet check valve (to keep fuel from re-
turning to fuel tank) and opens the outlet check valve, forc-
ing fuel to the carburetors.
3B-6 - FUEL SYSTEMS
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 occures these
valves can create operational problems and/or powerhead
damage by restricting flow of fuel. Some symptoms of re-
stricted (lean) fuel flow, are:
1 - Loss of fuel pump pressure
2 - Loss of power
3 - High speed surging
4 - Preignition/detonation (piston dome erosion)
5 - Outboard hesitates upon acceleration
6 - Outboard runs rough
7 - Outboard quits and cannot be restarted
8 - Outboard will not start
9 - Vapor lock
Any type of anti-siphon device must be located between
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) re-
place it with a solenoid-operated fuel shutoff valve.

Testing

Install clear fuel hose(s) between fuel pump and carbure-
tor(s). Run engine, and inspect hose(s) for air bubbles. If
air bubbles are found, see "Air Bubbles in Fuel Line" . If air
bubbles are NOT found, see "Lack of Fuel Pump Pres-
sure".
90-817643R1 DECEMBER 1996