Fuel Pressure Issues

[1972BlueC20]

Ok guys, some of you have been seeing my posts over the weeks and months and may know that I was having some issues with fuel pressure and some initial WOT hesitation more recently.

I adjusted the accelerator pump first and had no success with the hesitation. I had a new accelerator pump ready to install but first I wanted to try a regulator.

I installed a fuel pressure regulator today and set the PSI to 5.5 since I'm running an Edelbrock and it seems that the hesitation is now gone. I only drove it once for about 15 minutes and did several cruise to WOT and dead stop accelerations and it never hesitated. It actually felt like it had more power and was running much better having the pressure regulated.

Is that possible? Less fuel pressure would = more power and performance? Excuse that dumb question, but I'm learning.

The only problem that I now have left which I was hoping would be fixed with the addition of the regulator but wasn't, is the issue that after the truck sits after running and the pressure needle drops back down to 0 after the fuel runs back down the line.....it's hard to start.....you have to stand on the ignition for about 5 seconds before the fuel makes its way back up to the carb.

If you turn the truck off even if the engine is scolding hot, and the pressure is still showing on the gauge to be anymore than 0...whether its immediately after you turn the truck off or even 10 minutes after, then it fires right up immediately.

Once the pressure in the line drops to 0 PSI = hard start.

Is that normal to have hard start after sitting? How can I fix it?

Whats interesting is it starts right up every morning when its cold. Even though the pressure is at 0 in the morning.....I do however give it a squirt before cranking but it always fires right up.....giving it a squirt when hot after the pressure drops to 0 doesn't work during the day like it works in the morning. Its driving me crazy.

Also is it normal that when I rev the engine with my hand while watching the fuel pressure gauge that the pressure dips down a little on throttle?


Sorry I know that was long and is a lot of questions!

I appreciate your time and patience with me!!

[mechanicalman]

Originally Posted by 1972BlueC20

Ok guys, some of you have been seeing my posts over the weeks and months and may know that I was having some issues with fuel pressure and some initial WOT hesitation more recently.

I adjusted the accelerator pump first and had no success with the hesitation. I had a new accelerator pump ready to install but first I wanted to try a regulator.

I installed a fuel pressure regulator today and set the PSI to 5.5 since I'm running an Edelbrock and it seems that the hesitation is now gone. I only drove it once for about 15 minutes and did several cruise to WOT and dead stop accelerations and it never hesitated. It actually felt like it had more power and was running much better having the pressure regulated.

Is that possible? Less fuel pressure would = more power and performance? Excuse that dumb question, but I'm learning.

The only problem that I now have left which I was hoping would be fixed with the addition of the regulator but wasn't, is the issue that after the truck sits after running and the pressure needle drops back down to 0 after the fuel runs back down the line.....it's hard to start.....you have to stand on the ignition for about 5 seconds before the fuel makes its way back up to the carb.

If you turn the truck off even if the engine is scolding hot, and the pressure is still showing on the gauge to be anymore than 0...whether its immediately after you turn the truck off or even 10 minutes after, then it fires right up immediately.

Once the pressure in the line drops to 0 PSI = hard start.

Is that normal to have hard start after sitting? How can I fix it?

Whats interesting is it starts right up every morning when its cold. Even though the pressure is at 0 in the morning.....I do however give it a squirt before cranking but it always fires right up.....giving it a squirt when hot after the pressure drops to 0 doesn't work during the day like it works in the morning. Its driving me crazy.

Also is it normal that when I rev the engine with my hand while watching the fuel pressure gauge that the pressure dips down a little on throttle?


Sorry I know that was long and is a lot of questions!

I appreciate your time and patience with me!!

It will be interesting to see if the hesitation is really gone. The only way the fuel pump pressure regulator could have had an impact is if the pressure was blowing the inlet needle off the seat (over-powering the float) and spilling fuel out the top of the carb through the vent tube/s causing an over-rich mixture. You should have been able to see black smoke if that was happening.

I forgot why you didn't use the carb that came with the engine. Is that an option?

Whether the engine is hot or cold, it obviously has fuel in the bowl more than adequate to start the engine (the engine was hot when you shut it off, then started when cold so fuel didn't dis-appear then mysteriously appear in the bowl cold). How do you know fuel is running back down the fuel line?

After sitting over night, remove the air cleaner and see if the accelerator pump will put out more than one squirt. If it is able to put out 2-3 full squirts, I would say the fuel bowls are full enough.

The fact that it still starts hot only before the pressure goes to zero then doesn't start, but somehow fires right up cold (cold requires more fuel) makes no sense to me at all. Enough fuel in the bowl to start it hot if enough in the bowl to start it cold. Yet, you relate this to the 0 fuel pressure. You got me stumped hope somebody can make sense of this for you.

If it were inadequate spark (poor power supply to the coil), it would not start well hot at all and having more than 0 fuel pressure would not make a difference.

Something is missing in this picture.

[Fitz]

Originally Posted by mechanicalman

It will be interesting to see if the hesitation is really gone. The only way the fuel pump pressure regulator could have had an impact is if the pressure was blowing the inlet needle off the seat (over-powering the float) and spilling fuel out the top of the carb through the vent tube/s causing an over-rich mixture. You should have been able to see black smoke if that was happening.

I forgot why you didn't use the carb that came with the engine. Is that an option?

Whether the engine is hot or cold, it obviously has fuel in the bowl more than adequate to start the engine (the engine was hot when you shut it off, then started when cold so fuel didn't dis-appear then mysteriously appear in the bowl cold). How do you know fuel is running back down the fuel line?

After sitting over night, remove the air cleaner and see if the accelerator pump will put out more than one squirt. If it is able to put out 2-3 full squirts, I would say the fuel bowls are full enough.

The fact that it still starts hot only before the pressure goes to zero then doesn't start, but somehow fires right up cold (cold requires more fuel) makes no sense to me at all. Enough fuel in the bowl to start it hot if enough in the bowl to start it cold. Yet, you relate this to the 0 fuel pressure. You got me stumped hope somebody can make sense of this for you.

If it were inadequate spark (poor power supply to the coil), it would not start well hot at all and having more than 0 fuel pressure would not make a difference.

Something is missing in this picture.

His carb is an Edelbrock...notorious for their sensitivity to fuel pressure. At an RCH over 6 psi you DO blow the needle off the seat. The mixture was likely adjusted too lean to overcompensate so...add a regulator and Voila....won't start because it's now too lean. Gewt out a vacuum guage and reset the mixture, read the plugs and repeat until it's right.

[mechanicalman]

Originally Posted by Fitz

His carb is an Edelbrock...notorious for their sensitivity to fuel pressure. At an RCH over 6 psi you DO blow the needle off the seat. The mixture was likely adjusted too lean to overcompensate so...add a regulator and Voila....won't start because it's now too lean. Gewt out a vacuum guage and reset the mixture, read the plugs and repeat until it's right.

He did add a regulator, and is set at 5.5 psi.

I agree to re-set the idle mixture and give it a go. But why does he have to wait until it goes to 0 fuel pressure for the problem to exhibit itself? The devil is in the details.

[mechanicalman]

Originally Posted by 1972BlueC20

Ok guys, some of you have been seeing my posts over the weeks and months and may know that I was having some issues with fuel pressure and some initial WOT hesitation more recently.

I adjusted the accelerator pump first and had no success with the hesitation. I had a new accelerator pump ready to install but first I wanted to try a regulator.

I installed a fuel pressure regulator today and set the PSI to 5.5 since I'm running an Edelbrock and it seems that the hesitation is now gone. I only drove it once for about 15 minutes and did several cruise to WOT and dead stop accelerations and it never hesitated. It actually felt like it had more power and was running much better having the pressure regulated.

Is that possible? Less fuel pressure would = more power and performance? Excuse that dumb question, but I'm learning.

The only problem that I now have left which I was hoping would be fixed with the addition of the regulator but wasn't, is the issue that after the truck sits after running and the pressure needle drops back down to 0 after the fuel runs back down the line.....it's hard to start.....you have to stand on the ignition for about 5 seconds before the fuel makes its way back up to the carb.

If you turn the truck off even if the engine is scolding hot, and the pressure is still showing on the gauge to be anymore than 0...whether its immediately after you turn the truck off or even 10 minutes after, then it fires right up immediately.

Once the pressure in the line drops to 0 PSI = hard start.

Is that normal to have hard start after sitting? How can I fix it?

Whats interesting is it starts right up every morning when its cold. Even though the pressure is at 0 in the morning.....I do however give it a squirt before cranking but it always fires right up.....giving it a squirt when hot after the pressure drops to 0 doesn't work during the day like it works in the morning. Its driving me crazy.

Also is it normal that when I rev the engine with my hand while watching the fuel pressure gauge that the pressure dips down a little on throttle?


Sorry I know that was long and is a lot of questions!

I appreciate your time and patience with me!!

If you want to rule out ignition, get some 10-12 ga wire, a good female spade connector on one end, and a heavy alligator clip on the other. Put the truck in a situation where it's going to be hard to start (engine hot, fuel pressure 0).

Un-plug the power wire on the HEI and run the jumper from the HEI power inlet to the hot side of the battery. If that don't help, keeping the engine warm and letting the pressure go back down to 0, do it again with the same wire plus an extra ground wire to the engine.

It obviously has a good enough ground to crank the engine, but remember you are cranking it and trying to feed the ignition at the same time so a poor ground could be enough to crank it but not enough to supply a good circuit to the coil. You have to have good ground to achieve proper voltage.

[1972BlueC20]

The reason I'm not currently using the Holley 670 carb that came with the engine is because the mechanic who did my engine swap said he couldn't get it running right. He tried to say the carb was defective. I'm not buying that. I might try to put the Holley back on again and see how it runs. I do know first hand it was indeed running bad with the Holley but I think it might have been because I was running a Holley mechanical pump that was putting out about 8 psi and I wasn't running a regulator at that time. Now that I have a Lower psi pump and regulator thats set to 5.5 I think it might act better. I just really like the Edelbrock because they are very good for daily drivers and they seem to get better mpg. I have a # 1406.


As for why I think the fuel is running back down the line it's because after the truck sits and the gauge shows 0 Psi you can take the fuel line off the feed line and nothing comes out but a few drops. If you try that with the pressure still reading it squirts all over the place.

[mechanicalman]

Originally Posted by 1972BlueC20

As for why I think the fuel is running back down the line it's because after the truck sits and the gauge shows 0 Psi you can take the fuel line off the feed line and nothing comes out but a few drops. If you try that with the pressure still reading it squirts all over the place.

Is the choke all the way open when the engine is hot after the fuel pressure goes to zero? Maybe by that time it's started to close and is too rich/hard to start hot.

Mark where you have your choke now so you can go back, but try leaning the choke out a little maybe.

Maybe a phenolic carb base adaptor would help, I suppose it's possible for the fuel in the bowls to be getting too hot but I've stayed away from that because it starts right up cold. I suppose some of it could be vaporizing allowing the float to sink and draining the fuel back into the line and down. This would explain the 0 fuel pressure phenomenon and the vapors maybe flood the engine though vapors usually light off easy.

After engine hot, fuel pressure 0, crank it but not till it starts yet. Remove a plug and see if it's wet.

Something like this with 4 individual holes, and 4 hole gaskets. If you went open adaptor you would lose some responsiveness. Comes with gaskets and studs. I'd go no thicker than 1/2" unless you are sure your cables and linkages will still reach.
http://www.summitracing.com/parts/edl-8723/overview/

[1972BlueC20]

Good points, I'll look into those things!

What's your opinion on plastic phenolic spacers vs the wood fiber one you posted a link to?

[mechanicalman]

Originally Posted by 1972BlueC20

Good points, I'll look into those things!

What's your opinion on plastic phenolic spacers vs the wood fiber one you posted a link to?

I like wood, but only because I like wood and it's small impact on the environment. Not sure if one is better than the other, they all seem to work well on kitchenware.

[mechanicalman]

Originally Posted by 1972BlueC20

The reason I'm not currently using the Holley 670 carb that came with the engine is because the mechanic who did my engine swap said he couldn't get it running right. He tried to say the carb was defective. I'm not buying that. I might try to put the Holley back on again and see how it runs. I do know first hand it was indeed running bad with the Holley but I think it might have been because I was running a Holley mechanical pump that was putting out about 8 psi and I wasn't running a regulator at that time. Now that I have a Lower psi pump and regulator thats set to 5.5 I think it might act better. I just really like the Edelbrock because they are very good for daily drivers and they seem to get better mpg. I have a # 1406.

AVS style Edelbrock Thunder series are the only carbs I like better than the Quadrajets, then AFB style Edelbrock (1406 for example). Quads and Thunders have Adjustable Valve Secondaries, AFB styles can be adjusted but more difficult as you saw in the article you read.

As I remember, you did your homework on the engine installation and you made sure that the shop you used was approved for GM warranty on that engine.

So I'm wondering why the mechanic that said the carb was bad did not contact GM and get you a new one?

You are STILL under warranty. I recommend you take that completely drained carb back to the shop nicely packed in a shipping box ready to go just not taped and request (demand) they file the warranty claim. They WILL be compensated by GM, not only for the carb but for the labor required to install it. They have already done the diagnosis and a carburetor change out, so it's not dis-honest in reality just mis-dated. Tell them "just get me the carb I paid for, make out an R.O. that says you replaced the carb, I'll sign it, don't open the box just give me the carb".

If they insist on actually doing the work, remind them you are not paying ANYTHING and let them install it. I'd prefer the new carb in the box.

Like you, however, I would still prefer the AFB. But it would be nice to have a GOOD 670 Holley carb that YOU PAID FOR. And, what good is a BAD Holley carb? A new one still in the box wrapped with plastic is MUCH easier to sell, or put on the shelf for a rainy day.

BTW, you can go down a little more on fuel pressure if you like, say 5 psi. I've never ran jacked up pressure on an AFB but I tend to believe everybody that says 5.5 psi the max they can stand. I'd stay off the edge, you could play with it by turning it down until it starves for fuel then turn it back up only as far as you need to then another 1/2 lb. (I'm OCD). Yeah, I know, hard to believe LOL

[Lee H]

Couple of things, it's not abnormal for the pressure to bleed of after a while. Even when the pressure bleeds off the bowls are still full. 5 seconds of cranking is not way out of line for an old school carbureted engine. Not unusual for it to start differently cold VS hot. Do you pump the gas pedal before or while starting? What type of choke and how is it set up?

[1972BlueC20]

I have electric choke and it seems to be working great as it's been in the 30's and 40's here recently and it starts no problem.

In the mornings, I give it 1 or 2 pedal presses and it always fires right up.

During the day I just turn the key and after 5-8 seconds it fires up but stumbles a little I'm assuming til the fuel starts flowing normally.

I've tried pumping the pedal once or twice when hot during the day for restarts but it makes it worse and then you can smell fuel real bad after it fires up.

[Lee H]

OK, carbed engines need to be fed some gas while starting, hot or cold. Some engines need a little more, some a little less depending on carb, ignition timing, hot, cold, and ambient temperature. I'm not convinced you have a problem, maybe it's your new set ups personality. Maybe try just a little gas while cranking hot, maybe try gently putting pedal to floor and leaving it there while cranking, some where in between? Does your pump have a return line back to the tank?

[1972BlueC20]

No return line on pump. My tank has a return inlet but I just capped it off at the tank. I have a vented gas cap.

I'm happy with my truck now, I've figured out most of the issues. Just wanted it to start right up if that's possible. But I can live with the way it is if that's normal for this truck and it's set up.