I have a 455 Olds. It is in my Jet Boat, not in a car.
i am having a fuel issue.
when the engine sets for a week or more and even sometimes a day or two, the fuel in the upper fuel line between the pump and carburator, drains down.
It wont pump fuel up to carb by just cranking over.
i have to fill the primary fuel bowl and get the engine to fire off and run on fuel in bowl, for a few seconds, before fuel actually pumps up and keeps fuel bowl full and running.

it has a new fuel pump. It is a stock replacement pump. Has two nipples on top and one if not capped off pumps fuel out. Was told that is a return line to tank and is normal?

when the engine starts and runs for a few seconds, you can see it start pumping to carb, through clear filter before carb.

750 Demon carb.
Fuel regulator set at 7psi
stock mech fuel pump

every thing else works fine, just no fuel after sitting for a few days.
any ideas? Never experienced this particular problem before.

 

Sounds like you have a leaky check valve in the pump.

 

Why did you buy a fuel pump with a return line? Just get one without the return line so there's no worries about the cap on the return port leaking.

 

Hmm, i wasn't aware that these diaphragm pumps, had a check valve in them.
that could make some sense.

 

It was the only stock replacement, fuel pump, that the auto parts store, had to offer. I wasn't aware at the time that it was a return line nipple.
Would a bad seal on the cap cause a leakdown?