Hi,
I put in a new mechanical (carter) fuel pump when the diaphram rotted in my other one, and now I am just getting low fuel pressure.
When I start it up cold, I get maybe 5-1/2 pounds, then seems to drop to zero in about 3-4 minutes. The glass sight tube on the filter is nearly full when at the higher pressure, but then lots of air when low pressure.
I have a filter before the pump, the pump has a filter canister, and the glass filter on the pressure side.
Sounds to me like a filter, but they are all new, so don't think that would be the problem.
Couple of questions:
Is there a filter screen in the tank - maybe plugged?
What should the stock pressure be coming from the pump - without pressure regulator (took mine out and didn't change anything)

69 F100, 390, C6, DNE, Detroit Locker 3.73

 

about 3-4lb is about all that the stock pumps ever make, no screen on the pickup tube,

 

D1AZ-9A011-A .. Plastic Meshed Filter Screen (aka 'sock filter') - 3/8" I.D. / Available from Ford/autoparts stores / Slips onto the end of the fuel tank sending units pickup tube.

1961/79 FoMoCo Passenger Cars (except Maverick/Comet/Pinto Bobcat-use D1FZ-9A011-A 5/16" I.D.) / 1961/79 Econoline / 1966/79 Bronco / 1967/79 F100/350 (except 1967/77's with in-cab fuel tank).

In-Cab fuel tank sending units do not have a pickup tube as it's located on the tank itself. Auxillary fuel tank first offered in 1967.

 

I think ya may be sucking air. Well not you, But your pump.
A fitting or even a pin hole in an old line can do what your
system is doing. And ya may not even see any fuel leaking
other than maybe a damp spot on a rusty line or new fitting.
5.5 pounds on a new mechanical pump is about right.
The run of the mill inexpensive electrical pumps push about 7psi.

Another thing just came to me. Did ya change yer fuel cap?
If you put on a non vented cap or it is vented and plugged it
could also be your problem.

 

Thanks guys.

Mine has the in-cab fuel tank, so no sock. Thanks for the info ND.
Guess I'll start looking for air leaks. Probably very possible on 44 year old fuel lines!

 

Are ya sure the cap is vented?
Try it with the cap off just for the Halibut.

 

If the suction is submerged in fuel, and the pump is always pumping, how could air get into the filter?

That fact alone points to an air leak.

 

Well, worked on it this afternoon. Replaced the filter before the pump with a see-through type, just because it was only $3.99. The other filter was clean, I cut it open to look inside just to see if there was any rust or crud inside. Nothing.
My cap is a vented style, the gasket is in such poor shape that there is no way it would seal, but I did try with the cap off, no change.

When it idles, (after the pressure drops from about 4 to zero, it runs smooth for a few seconds, then idle drops off to almost stall, then smooth, then drops, then smooth. It just keeps it up like that as long as I want.

Air in the filter. I have had glass filters forever, and they almost always have some air in them. This one had a lot in it, but today just a large bubble at the top when it was acting funny.

I think it is something to do with the new pump. Didn't have this problem before I changed it. Checked all the connections to the pump, fuel lines, inspected the fuel line from the tank (looks great). Checked vaccuum to see if that had something to do with it, it is normal for the cam I am running - same as it always is.
I don't know, seems like maybe with very LITTLE pressure, maybe the gas in the bowl drops then doesn't recover fast enough at idle to keep it from being fuel starved. When the bowl inlet valve is wide open it refills and runs smooth till the level drops again? BTW - Holly 4150
Got me puzzled. Probably a Chinese pump - even though I went out of my way to get a Carter.
Alot of parts are getting pretty bad nowadays. I got 2 scat cams about a month ago, they weren't even marked right. The lobe profiles were for a completely different cam than the one I ordered, and was marked. Gotta check EVERYTHING any more, no more assumptions that things will be right. (unless of course you can get genuine OEM parts still.)

 

This worked for me. disconnect the supply hose off pump and blow air in it. You will know then if you have a leak somewhere causing the fuel pump
to suck air. If you can't blow into it then it's got to be in the seam of the fuel pump.