Here are a few thoughts on fuel pressure issues:
Normal fuel pump pressure is between 2 and 6 PSI, depending on the
application. Let's say 3 or 4 PSI for the GPW (just guessing).
If pump pressure is too high, here are some possible causes:
1) Whoever rebuilt the pump could have pulled the diaphragm too tight
during assembly. This is a VERY common cause of over-pressure fuel
pumps. There must be a certain amount of "slack" in the diaphragm.
During the pumping stroke, pressure does not build appreciably until
this slack is taken up. If the diaphragm has been pulled too tight, the
pressure starts building up earlier in the pump stroke. This compresses
the diaphragm spring farther than usual, and the further you compress
the spring, the greater the pressure.
2) The diaphragm spring could be too stiff.
3) There could be a rocker arm/linkage problem that prevents the
designed-in free travel, thus increasing the effective stroke length and
compressing the diaphragm spring further.
4) Actually, moving the fuel pump farther from the block CAN sometimes
reduce fuel pressure, since the fuel pump eccentric on the camshaft
would tend to contact the rocker arm farther from its pivot, which would
reduce the stroke length, which would reduce the amount of compression
on the diaphragm spring. HOWEVER, this can be a bit risky, since on some
vehicles it could create unwanted clearance between the rocker arm and
the low area on the eccentric, resulting in a "slapping" condition that
will increase wear. Or, it could cause the end/edge of the rocker arm to
rub against the eccentric. That said, on some vehicles you might get
away with it. The best approach, however, is to use a pump that produces
the correct amount of pressure in the correct mounting location.
5) Wrong pump.
Some people install a fuel pressure regulator valve in the fuel line
between the pump and the carburetor, but this should not be necessary
with a properly functioning fuel pump that is correct for the fuel
system.
Finally, even though excessive fuel pressure CAN hold a float valve open
or raise the fuel level in the bowl, it is ALSO common to see improperly
adjusted float valves, deteriorated synthetic rubber tips on needle
valves, debris in a float valve that holds it open, varnish/glop in the
needle valve that makes it stick, or a bent float assembly that rubs
against the carburetor body.
Test the fuel pressure. If it's too high, get a properly rebuilt pump.
Hope that gives you some ideas.
Alan
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