Puzzling Mechanical Fuel Pump Problem....
 

Ok so I'm trying to fire up this 71 Olds today that I've been working on. Fresh motor with all new accessories, etc. It's a 455. Well for some reason the fuel pump isn't pulling fuel from the tank.

If I remove the inlet hose and stick it in a cup of fuel it pumps it no problem. If I suck on the line coming out of the tank I get a mouthfull of gas (threw that right back up). So the pump pumps and the line flows....yet when I put the two together I get nothing. Anyone have any idea? Pump not strong enough to pull fuel all the way from the tank??

Is the gas cap on? Kind of a stupid question, I know.

Tried it with and without it on....but not sure how it would make a difference.

Fixed it....forced about 80 psi of air back through the supply and it cleared out some sort of obstruction.

I was kinda just thinking out loud. Not sure how it is on that Olds, but I'm fairly certian that on some pre-emissions stuff a properly working vented cap is needed to pull gas from the tank- otherwise a vacuum will form. That would leave the only thing blocking it being a bad sending unit sock, which you probably took care of with that air. It may have been clogged enough to pull shut when the pump was pulling, but couldn't pull shut when you were trying to draw at a lower vacuum. Was the tank/sender new?

I thought of the vac problem as well Bill which was why I pulled the cap...but I thought I was being nuts.

Tank is original and I didn't drain it....no plug and customer didn't want to pay for me to pull it out and dump it. Seems to be pumping now but I blew a radiator hose and the motor was super hot during break in. Hoping it's a timing issue and nothing got hurt. Weird because it seemed to be running fine til the hose blew. Of course the thing only had idiot lights so I couldn't monitor anything all that well.

If it was the original sock from '71, it may have blown apart...

Very highly possible....sometimes it's hard to realize I'm running a business and can't spend time and money out of my own pocket.