Not enough fuel pressure, lost hp.
Thread Starter
Melting Slicks
Joined: Sep 2004
Posts: 2,412
Likes: 3
Not enough fuel pressure, lost hp.
Got my Mighty Demon 750 dialed in, took it out for a drive... major disappointment. It feels like I lost 50+hp whereas I thought I'd gain about 50 considering that I was upgrading from a fuel starved stock 78 Q-Jet. Also the carb backfired once when I tapped the gas down low in gear.
Anyways my inline fuel pressure guage reads 4psi. The Carter fuel pump supposedly was going to give me 7psi and 120gph. I'll be returning that piece of crap. I'm assuming that this is my problem, what do you guys think? Any recommendation for a new mechanical fuel pump? Or do I need to go with an electric pump?
As far as adjusting the carb, I backed the idle mixture screws out 2 full turns, and I moved the idle speed screw out 3/4 of a turn from the base setting (about .02" of the transfer slot showing).
Final question, I noticed that while running the clear fuel filter fills up with more air than it does fuel, could my new fuel filter be restricting flow? Could I damage the carb if I removed the fuel filter for a few minutes to check this theory?
Anyways my inline fuel pressure guage reads 4psi. The Carter fuel pump supposedly was going to give me 7psi and 120gph. I'll be returning that piece of crap. I'm assuming that this is my problem, what do you guys think? Any recommendation for a new mechanical fuel pump? Or do I need to go with an electric pump?
As far as adjusting the carb, I backed the idle mixture screws out 2 full turns, and I moved the idle speed screw out 3/4 of a turn from the base setting (about .02" of the transfer slot showing).
Final question, I noticed that while running the clear fuel filter fills up with more air than it does fuel, could my new fuel filter be restricting flow? Could I damage the carb if I removed the fuel filter for a few minutes to check this theory?
Thread Starter
Melting Slicks
Joined: Sep 2004
Posts: 2,412
Likes: 3
Originally Posted by big_G
I think you under-estimated the Q-Jet and over-estimated the Demon, IMO. The carb. will not lose power, only jetting/fuel flow will. 
Race Director
Joined: Sep 2004
Posts: 11,874
Likes: 263
From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by torqvette
.... Also the carb backfired once when I tapped the gas down low in gear.....
Anyways my inline fuel pressure guage reads 4psi. The Carter fuel pump supposedly was going to give me 7psi and 120gph. . I'm assuming that this is my problem, what do you guys think? ......As far as adjusting the carb, I backed the idle mixture screws out 2 full turns, and I moved the idle speed screw out 3/4 of a turn from the base setting (about .02" of the transfer slot showing).
Final question, I noticed that while running the clear fuel filter fills up with more air than it does fuel, could my new fuel filter be restricting flow? Could I damage the carb if I removed the fuel filter for a few minutes to check this theory?
Anyways my inline fuel pressure guage reads 4psi. The Carter fuel pump supposedly was going to give me 7psi and 120gph. . I'm assuming that this is my problem, what do you guys think? ......As far as adjusting the carb, I backed the idle mixture screws out 2 full turns, and I moved the idle speed screw out 3/4 of a turn from the base setting (about .02" of the transfer slot showing).
Final question, I noticed that while running the clear fuel filter fills up with more air than it does fuel, could my new fuel filter be restricting flow? Could I damage the carb if I removed the fuel filter for a few minutes to check this theory?
ok heres my thoughts...backfiring at low speeds classic signs of mis adjustment....your thoughts on that fuel pump may be correct....idle mixture screws are out way too far.....and i bet this is because you are not getting enough fuel for the carb to act right and you are over compensating by making the mixture too rich....where are the floats set to?....as far as the fuel filter low pressure gas systems always seem to do that ....in one of my british cars you never saw gas in the fuel filter and when i mentioned it to my mechanics they said to put tape over it so it wouldn't bother me.....
Thread Starter
Melting Slicks
Joined: Sep 2004
Posts: 2,412
Likes: 3
Originally Posted by bobs77vet
ok heres my thoughts...backfiring at low speeds classic signs of mis adjustment....your thoughts on that fuel pump may be correct....idle mixture screws are out way too far.....and i bet this is because you are not getting enough fuel for the carb to act right and you are over compensating by making the mixture too rich....where are the floats set to?....as far as the fuel filter low pressure gas systems always seem to do that ....in one of my british cars you never saw gas in the fuel filter and when i mentioned it to my mechanics they said to put tape over it so it wouldn't bother me.....
Also, what carb were you using before the Speed Demon? How big of an acceleration difference was it?
Drifting
Joined: Sep 2004
Posts: 1,959
Likes: 1
From: North Bend Ohio
Backfiring through the carburetor is a lean condition. Did you verify the float level? 4 PSI of fuel pressure would be enough to keep the float level at the required amount during normal driving. If the float level is correct when the backfire takes place it cannot be the fuel pump causing it.
I would go to NAPA and get a stock fuel pump. The one you have may or may not be defective, but I don't believe your engine requires 1440 HP worth of fuel flow.
As far as the lack of power and backfiring go, first make sure the timing is correct. Retarded timing can cause this. You also need to check the operation of the accelerator pump. Make sure you're getting the full stroke and volume out of it. You could open up the shooter to deliver the fuel faster.
BigBlockk
Later.....
I would go to NAPA and get a stock fuel pump. The one you have may or may not be defective, but I don't believe your engine requires 1440 HP worth of fuel flow.
As far as the lack of power and backfiring go, first make sure the timing is correct. Retarded timing can cause this. You also need to check the operation of the accelerator pump. Make sure you're getting the full stroke and volume out of it. You could open up the shooter to deliver the fuel faster.
BigBlockk
Later.....
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Race Director
Joined: Sep 2004
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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by torqvette
That would make sense, it seemed like the idle mixture screws wanted to be backed out even more. But the tuning video recommended 1 1/2 turns, so I didn't want to push it more than 2. That would also point me in the direction of not enough pressure. Thanks again for the advice.
Also, what carb were you using before the Speed Demon? How big of an acceleration difference was it?
Also, what carb were you using before the Speed Demon? How big of an acceleration difference was it?
Race Director
Joined: Jul 2001
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Originally Posted by bobs77vet
...and i bet this is because you are not getting enough fuel for the carb to act right and you are over compensating by making the mixture too rich..
Instructor
Joined: Dec 2005
Posts: 135
Likes: 39
From: Chardon Ohio
Sounds tuning-related; not fuel pump/pressure related. The pressure doesn't equate to power. As long as your pump can keep up with fuel demand, you're good to go.
Also, I'd take a good long look for vacuum leaks, possibly between the baseplate and the manifold.
Also, I'd take a good long look for vacuum leaks, possibly between the baseplate and the manifold.
Last edited by Vicious_Cycle; Jan 13, 2006 at 08:22 AM.
Melting Slicks
Joined: Mar 2004
Posts: 3,472
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From: Lake Ozark, MO
As far as adjusting the carb, I backed the idle mixture screws out 2 full turns, and I moved the idle speed screw out 3/4 of a turn from the base setting (about .02" of the transfer slot showing).
Mike
Race Director
Joined: Sep 2004
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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Originally Posted by Mike Ward
If he's not getting enough fuel , how could he overcompensate making it too rich?
Tech Contributor
Joined: Aug 1999
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From: At my Bar drinking and wrenching in Lafayette Colorado
4psi is plenty of fuel pressure - pressure is not the problem.
If you have a hesitation or a backfire, you're running lean. You need to set up the carb for your application with correct jetting and adjustments. This will dramatically change throttle response.
If you have a hesitation or a backfire, you're running lean. You need to set up the carb for your application with correct jetting and adjustments. This will dramatically change throttle response.