new engine getting only 3 psi fuel pressure

[1972BlueC20]

Just got my truck back from getting a new engine installed part # 19210008.

It has a Holley 670 vacuum secondary carb and a Holley mechanical fuel pump.

When I first crank the truck it has between 6-7 psi of fuel pressure (gauge is on dual feed line near carb). After the truck idles for a while the pressure drops to between 3-4 psi. I can also see bubbles start to come up in the fuel filter after the pressure drops.

I cant see any leaks in the lines (rubber 3/8 line from tank to pump and from pump to filter and carb)

I thought maybe my tank needed a vented cap. Currently it doesn't have a vented cap. My truck is a c20 non eec tank. It has a return line for the original style pump with 3 ports. This new Holley pump has only 1 inlet, so I capped the return tube at the tank itself. After the truck idled and once the pressure dropped I took the gas cap off and the pressure stayed the same so I don't think it's the cap that's causing the issue. Also the sending unit and sock is new in the tank.

I think the pressure should be staying at between 6-7 psi at all times correct?

Anyone have any idea why I'm getting only 3 psi after idling for 10-15 minutes??

[cdowns]

with all thast rubber fuel line just use it till it burns to the ground and then start all over

[1972BlueC20]

Thanks for answering my question.

[hotrod farm truck]

3 -4 psi is correct for a stock fuel pump. Shouldn't be a problem.

[Sammy]

I am almost sure Holley's require at least a constant 6 to 7 PSI unlike some other carbs. My small block maintains that according to the gauge.

[Jarrett1967]

He's right about the rubber fuel line - it is not safe and it is not expensive to do it correctly, so why risk it?

[1972BlueC20]

Originally Posted by Sammy

I am almost sure Holley's require at least a constant 6 to 7 PSI unlike some other carbs. My small block maintains that according to the gauge.

Yeah that's what I thought. Any ideas as to why the pressure would be low?

[Sammy]

Wonder if a pressure regulator would cure it? Someone surely can provide insight.

[leddzepp]

If the truck is running fine I wouldn't worry about it.

[1972BlueC20]

Originally Posted by Sammy

Wonder if a pressure regulator would cure it? Someone surely can provide insight.

If the pressure is already low coming directly from the pump, would a regulator be able to raise the pressure? I wouldn't think so. I may be wrong.

[picklito]

A different thought about that rubber fuel line... could it be kinked somewhere? Or, if there's a restriction somewhere in the flow from the tank, it might be collapsing (not gonna beat you up about the safety thing, but I'm hoping this gives you another reason to install hard lines).

[FirstOwner69]

Originally Posted by 1972BlueC20

Yeah that's what I thought. Any ideas as to why the pressure would be low?

According to the Holley Street Avenger instructions (http://documents.holley.com/199r10219-3rev4.pdf, page 7) the recommended fuel pressure is 5-7 PSI.

You said "After the truck idles for a while the pressure drops to between 3-4 psi." Is the engine at the same RPMs before and after the pressure drop? Don't know if that would make a difference in pressure.

What Holley mechanical fuel pump do you have?

Just a wild thought, but there have been a couple negative comments about the rubber fuel line. That aside, is it possible that the rubber line between the pump and tank may be collapsing from trying to draw fuel too fast? I'll let the experts comment on that.

[raveer2000]

Really should get 2 long pieces of brake tubing from any parts house and only use short pieces of rubber line on each end.

[Sammy]

Some research I did on Holley's claim no more than 7 PSI to prevent flooding. Guess that would be a maximum pressure. But no agreed indication of what requirements on the low side depending on where you are educating yourself. If you do a search on the web there are lots of opinions from cam lobe specs, rubber line issues to even 1/2" lines on various applications on big block applications.

[Jarrett1967]

Last comment on the rubber lines - if they are old, it is possible you've disturbed a pin hole leak while installing the new sending unit hardware? Pin Hole leaks, collapsing rubber lines, and clogged filters all are capable of causing the bubbling that you see after idling. The fuel pump starts pulling air in with the fuel somewhere. Any chance a filter is backwards, restricting fuel? The fact that you see 6-7 psi makes me think your fuel pump/cam combo should be fine. The low pressure only starts to happen once it runs out of the initial fuel, and starts sucking air. Once you do find your problem - then a regulator could be added to help stabilize pressure at the carb (if you need it) and also to prevent blowing the needle off the seat in the case of too much pressure (over 7psi). Good luck - hope you find it.