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Old 01-31-2007, 10:46 PM
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Fuel or Ignition Problem?

I can't seem to find anything that really helps my particular problem through searching the posts. Here's the deal. I recently rebuilt a 350 for my 1990 Chevy. I done away with the
TBI. .30 over, new crank, pistons, the works. Mild computer compatible cam. Adv. Duration: 266�/272�
Dur. @ .050'': 210�/216�
Lift: .440''/.454''
Lobe Sep.: 114�
Weind Action Plus Intake.
New Vortec heads
Roller rockers 1.5 ratio
Fresh Quadrajet
HEI

The truck starts and runs very good and accelerates nicely through 1st and 2nd gear. (5 speed manual). But in 3rd or 4th at mid to upper range rpms, it acts as if it's not getting enough fuel and bogs, jerks and surges. If I let off the throttle a little it smooths out. It don't occur when accelerating from 1, 2. It raps out nice. I am still using the in tank electric fuel pump with a bypass fuel regulator that utilizes the return line. I've adjusted the pressure up as high as it would go and the same thing. However, I've adjusted it as low as it would go with very little difference noticed. I'm pretty sure the valves are set correctly, I can't find a vacuum leak, the ignition is new and stock. I can't figure it out. It runs smooth and strong except at 1/2 throttle in 4th gear mainly. It has to be something stupid or simple. Any ideas would be greatly appreciated.

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Old 02-01-2007, 08:11 AM
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sure sounds like lack of fuel.
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Old 02-01-2007, 03:12 PM
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I wonder if the load in 4th gear at 1/2 throttle is fluttering the secondary's.

Tighten it up and see.

Or zip tie the air door shut and try it.
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Old 02-01-2007, 04:10 PM
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I tried wiring the air door shut already. and open for the heck of it and the same
thing. This is the second freshly rebuilt quadrajet i've tried and they both performed the same with the exception of the current carb idles much smoother and starts and idles fine when cold in below freezing temps. I've adjusted and readjusted the jets, played with the timing, and gave it as much fuel pressure as
I could. Maybe the fuel pressure regulator is defective. It's brand new (which doesn't mean much). When turned to maximum psi, the carb doesn't flood or run crappy. When turned to minimum psi, I can't tell a difference. I don't want to buy another $80.00 regulator. I guess I could install a mechanical pump. But
I hear it's better to pull the electric one to get max flow. The fuel pump I'm sure is functioning properly, the tbi ran fine with it. I'm pullin my hair out.
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Old 02-01-2007, 04:19 PM
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Presure doesnt mean volume, you still could be running out. If you can get a presure gauge with a fairly long hose, like the snap-on style, you will be able to monitor fuel presure when you are having the issue. I would also make sure you didnt crimp a line or something that could restrict flow. If I remember correctly the TBI setup only pushes about 8 or 9 LBS.
You may want to try temporaraly block off the return to see if the issue goes away, that will prove if the regulator is working or not. Also check your venting for the tank to be sure it is working. Take it for a ride with the cap of and see if it is better.

Good Luck with it

Chet




Quote:
Originally Posted by grandfisher
I tried wiring the air door shut already. and open for the heck of it and the same
thing. This is the second freshly rebuilt quadrajet i've tried and they both performed the same with the exception of the current carb idles much smoother and starts and idles fine when cold in below freezing temps. I've adjusted and readjusted the jets, played with the timing, and gave it as much fuel pressure as
I could. Maybe the fuel pressure regulator is defective. It's brand new (which doesn't mean much). When turned to maximum psi, the carb doesn't flood or run crappy. When turned to minimum psi, I can't tell a difference. I don't want to buy another $80.00 regulator. I guess I could install a mechanical pump. But
I hear it's better to pull the electric one to get max flow. The fuel pump I'm sure is functioning properly, the tbi ran fine with it. I'm pullin my hair out.
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Old 02-01-2007, 04:24 PM
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You could have a fuel hose sucking together and cutting off the fuel supply when you get past half throttle.
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Old 02-01-2007, 09:36 PM
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Quote:
Originally Posted by Henry Highrise
You could have a fuel hose sucking together and cutting off the fuel supply when you get past half throttle.
Constant volume in tank pump pushing fuel. #1 post.
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Old 02-01-2007, 09:49 PM
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[QUOTE=grandfisher]I

The truck ....accelerates nicely through 1st and 2nd gear..... It raps out nice.

But in 3rd or 4th at mid to upper range rpms,..... bogs, jerks and surges.
If I let off the throttle a little it smooths out.

It runs smooth and strong except at 1/2 throttle in 4th gear mainly.


This is the second freshly rebuilt quadrajet i've tried and they both performed the same

QUOTE]

Does it jerk at WOT or partial throttle???

#1 Try it with the air filter OFF, bare carb.

#2 HEI new or used?

Weak coil, bad module

Last edited by xntrik; 02-01-2007 at 09:59 PM.
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Old 02-02-2007, 01:48 PM
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Define for us a jerk and surge,


I can't feel it over the net so get very descriptive.
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Old 02-02-2007, 04:05 PM
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Thanks for the replies. I found out my problem. The fuel pressure regulator I ordered came with a 90 degree elbow with a restriction port in it. I was going to
block off the return line as was suggested when I remembered the restriction fitting. So I switched the carb and return lines and problem solved. I knew it was something simple or stupid. It was both. I'll drill the restriction out to be sure the return is fully functional and I should be set. Thanks for the help.
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