So far, all of the advice has been good:
Possible ruptured diaphragm in old fuel pump (if fuel is still routed =
through the old pump).
Possible leaky float needle valve.
Fuel pump pressure too high. (Up to about 8 psi is probably OK. Beyond =
that, you should get a lower-pressure pump or a pressure regulator.)
Here are a couple of other things to check:
A bad gasket between the main body and the throttle body will allow excess =
fuel from the idle circuit to be sucked into the intake manifold when the =
engine is running.
The float needle valve could be leaking for the following reasons: float =
level maladjustment, sticking due to debris or bent parts, softened/dissolv=
ed rubber tip on needle, leak in one or both brass floats.
Newer Viton-tipped float needle valves are alcohol resistant and will work =
fine with oxygenated fuels. You can get one at NAPA (NAPA/Echlin 2-3516).
If you install a spring-loaded-type needle valve, you must put a .050-inch =
feeler gauge between the float lip and the needle valve when you set the =
float level to 5/64 inch (top of float to top of main body casting).
If you install a solid or viton-tipped needle valve, set the float level =
to 5/64 inch, but WITHOUT the feeler gauge in place.
If the float level is too high, excess fuel can be sucked down around the =
edge of the step-up piston directly into the intake manifold, particularly =
at idle or under low load when manifold vacuum is high. It will also cause =
the high-speed mixture to be too rich.
If the float valve leaks (doesn't shut off at all), there are several =
places where the fuel can be pumped directly into the carb or manifold.
This may sound odd, but it could even be due to a cracked casting. I found =
a used carb the other day that had a long hairline crack across the bottom =
of the fuel bowl. Needless to say, I didn't buy that one.
Someone in Military Vehicles Magazine was advertising M37 fuel pump repair =
kits. If the kit is new manufacture, it will probably be OK with alcohol-be=
aring fuels. You might just want to rebuild the mechanical pump and remove =
the electric pump (or plumb it as a backup pump that bypasses the original =
pump).
Regards,
Alan
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