Losing fuel pressure on new setup

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zinser72

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I just finished changing out the whole drivetrain in my 72 duster. I have a new 3/8 pickup, 3/8 hard line (aluminum) with the fuel filter roughly by the passenger floor. Running a carter m6270 if i remember right and a holley dead head style regulator with -6AN from the pump to the carb. Holley 750 dp with a proform main body, carb is proven and worked great last year.

So heres what its doing: it will start fine once fuel is in the bowls. I will have good pressure (6.5psi) at the carb and after about 15 mintues of driving it will be down to 2psi hardly registering on the gauge and the engine starves and dies.

I tried eliminating the regulator to see if that was the issue and it started off at 8.5 psi and fell down to 2 just like before but it seemed to last a little longer. Then i thought possibly vapor lock but it was only 65 degrees out and i hardly drove it. Also i insulated the lines whenever they get close to the header. I have a stock pump i can put on to rule out the pump being faulty. What do you guys think? Everything is new and this is a pretty inconvenient problem being consistantly stranded. Feeding 500 hp 390 and open to any suggestions. I would like to keep as much of my setup the same as possible. Thanks for any input!
 
If your fuel tank isn't vented,, it'll create enuff vacuum in the tank to stop/reduce the fuel flow..

Next time it happens,, take the fuel cap off, listening for a "whoosh" as the cap releases,, leave cap loose, and see if problem persists..

cheers
 
inertia said:
If your fuel tank isn't vented,, it'll create enuff vacuum in the tank to stop/reduce the fuel flow..

Next time it happens,, take the fuel cap off, listening for a "whoosh" as the cap releases,, leave cap loose, and see if problem persists..

cheers
Click to expand...

That's exactly what sounds like is happening.
 
Thanks for the quick replies. I think my vent is good because i blew 10psi into the vent tube to get fuel up to the carb. But i will see if it does it with the cap loose anyway. Any other things to try?
 
Leaving cap loose is "the" test. The vent routing is in the service book. Might be plugged. There's a valve at the end of the tube in the engine bay.
 
Tomorrow before work i will check out the vent system. But i was also wondering do you think the gas is the issue? Im in wisconsin and maybe were still getting rid or winter blend since it dips in the 40's over night. Im using 93 octane with 10% ethanol. Thats the best i can get around here. Someone tell me im being stupid so i can move on to the next thought...
 
the easiest thing to do is try another fuel pump or put in a mechanical , replace the filter , bypass the regulator. obviously the pump you are using is not working or something is restricting the fuel. there should be a constant 5.5 -6 psi to the carb, if the pump isn't delivering that , then you need to try something different. I have a tank vent , I plug it off in the winter for storage. I forget to unplug it sometimes , I run and drive the car , I never lose fuel pressure before I remember to unplug it. I do not have a vented cap. your gasoline is not going to cause a loss of fuel pressure. maybe you have a rubber line collapsing. just because parts are "new" does not mean they are "good". it only takes a few minutes to try another pump and run it from the tank to the carb with just a filter
 
Well now im completely stumped. Heres what i have tried:
-removed regulator
-changed back to stock mechanical fuel pump
-took the gas cap completely off when pressure was dropping
-inspected every inch of the vent and feed lines, not close to any exhaust and no pinch points or kinks

And also what i just did is disconnect the fuel line downstream of the filter. Since my filter is under the car by the dash the fuel syphoned out of the tank. So the fuel was going through almost the entire plumbing and the filter and i got about a gallon in a bucket in about a minute. It showed no signs of slowing down so i stopped. This was done with the gas cap on. I dont know what else to check, and im still losing pressure just idling for 10 min. It will almost immediately just creep down on the gauge til its at nothing after about 15 min. I dont even have to drive it for the pressure to drop. Im at a loss. Could there be something wrong with the carb? I dont see how that could possibly affect pressure but i have checked or changed everything else. Any help is appreciated on what to try next
 
Well it would about have to be the pump then right?

Just so you know, those mud wasps don't make kinks in vent lines. :)
If it's not the pump then there is a restriction somewhere you can't see.

Fuel type will not cause the issue you are describing.
You could run water through it and still have the right pressure.
 
A leak or sucking air at one of your connections?Maybe an old sock from your old pickup in tank blocking fuel?
 
where are you tapping for fuel pressure? I would try this, get a buddy. Cap fuel line off carb. Get some sort of fuel container and run a line from it to the carb inlet (raise above carb so it gravity feeds to carb like an I.V setup, matter of fact, that would be perfect, hook it to hood latch) run motor and keep eye on pressure. If the pump is working properly (I dont think it is) youll see no drop in pressure. If its a Edelbrock style mechanical pump, you cant take it apart and check the check valves inside. IIRC, there are 3, one pulling and 2 pushing. I had one that had the 2nd pusher come unseated. produced next to no pressure. You can also pressurize your fuel line to back flush it into the tank, that might explain a collapsed sock or just a plugged one. I took one out that was totally plugged with rust. I blew air into it and the sock split under the pressure.
 
This sounds to me like vapor lock, or some problem in the suction side causing a restriction. I wold tee in a vacuum / pressure gauge (test vacuum / fuel pump gauge) on the inlet side. What do you have for a pickup screen, is it the typical sock? New? Old?

Also see what the pump will pull for vacuum on the inlet side with only the gauge on the inlet. Should be at least 12" Hg

Look in the service manual. There are procedures.

Just FYI I'm a HUGE believer in vapor return systems. Running a bypass regulator accomplishes this, with no regulator, you can use a Wix 33040 or 33041 filter, which has a 1/4" hose fitting with a return orifice, built in. You of course need to build a return tube back to the tank. Factory (440/ hemi) cars used 1/4" but I would run 3/8
 
pishta said:
where are you tapping for fuel pressure? I would try this, get a buddy. Cap fuel line off carb. Get some sort of fuel container and run a line from it to the carb inlet (raise above carb so it gravity feeds to carb like an I.V setup, matter of fact, that would be perfect, hook it to hood latch) run motor and keep eye on pressure. If the pump is working properly (I dont think it is) youll see no drop in pressure. If its a Edelbrock style mechanical pump, you cant take it apart and check the check valves inside. IIRC, there are 3, one pulling and 2 pushing. I had one that had the 2nd pusher come unseated. produced next to no pressure. You can also pressurize your fuel line to back flush it into the tank, that might explain a collapsed sock or just a plugged one. I took one out that was totally plugged with rust. I blew air into it and the sock split under the pressure.
Click to expand...

I just realized i never tried this. I will be doing this next for sure just to rule out the carb possibly
 
If your pump is sucking air, this is exactly the symptom.Generally it will suck air at that little short jumper hose right at the tank.This will not show up as a leak because its above the liquid line. It will show up better at idle. Once the Rs are up the pump tends to draw more fuel than air and it takes longer to show up.The higher the Rs, the longer the time delay.At next morning start-up the air in the line may have migrated back to the tank, so most of the line may have fuel in it,and the pump fills the bowls and pressure comes up. As the fuel gets idled away, the pump again brings up the air, and the pressure drops.
I put double back-to-back, 180*offset gearclamps, on that little trouble maker, and change it out every couple of years.
It may not be THAT hose. It could be any hose on the suction side, as well as the in-tank pick-up(that part above the liquid line).
 
Well i swapped carbs to a 600? Cfm holley i had laying around. Dont know much about it other than its old and worked the last time i used it. The car ran without a hitch with the other carb so im going to throw a kit in my good carb and reset the float levels. The weird thing is my pressure still did the same thing. Went from 7 to 2. It seemed to take longer but it still dropped.

Any chance its a faulty gauge? The car ran and drove for 30 min without a problem

My next plan is to put the good pump and reg back on with this carb and see what the pressure does and how it runs. I will also add clamps on my rubber connections as suggested to be safe and see If it changes anything
 
zinser72 said:
Well i swapped carbs to a 600? Cfm holley i had laying around. Dont know much about it other than its old and worked the last time i used it. The car ran without a hitch with the other carb so im going to throw a kit in my good carb and reset the float levels. The weird thing is my pressure still did the same thing. Went from 7 to 2. It seemed to take longer but it still dropped.

Any chance its a faulty gauge? The car ran and drove for 30 min without a problem

My next plan is to put the good pump and reg back on with this carb and see what the pressure does and how it runs. I will also add clamps on my rubber connections as suggested to be safe and see If it changes anything
Click to expand...

I think you found your problem.

You said in your first post that the motor starved and died, but now its not? So whatever you changed made some sort of difference.

I would leave everything as is. Start car and drive it, drive hard, drive it normal, etc. If it doesn't die or sputter, pull over and check the fuel pressure. If it is low, I would suspect a faulty gauge.
 
It sucked the bowls dry and died the first time. Then it stalled and was dripping out a booster so im thinking sticky needle and seat in my 750 carb. Ill be swapping out the gauge and driving it again changing nothing else to see if thats the problem. My best guess at this point is a combination of a bad gauge and a needle and seat acting up at the same time. Ill try some more stuff and report back. I think im getting there finally, thanks for all the help so far!
 
Its finally fixed! I changed out the gauge and its running and driving fine, never moves from 7 psi with a stock unregulated pump. So all along it was just a combination of a faulty gauge and needles and seats acting up in the carb.
 
It sounds like your fuel filter is before the fuel pump. It is better to put it after the pump where the pump pressure can help push gas through it. Before the pump is gravity fed and the filter can cause restriction for the pump to suck it in....
 
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