Need Help Diagnosing a Fuel Delivery Problem - Team Camaro Tech
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post #1 of 12 (permalink) Old May 25th, 10, 11:08 AM Thread Starter
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Mark
 
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Need Help Diagnosing a Fuel Delivery Problem

I could use some advice with a problem Iím having with my 69 RS. Iíve been trying to fix what appears to be a fuel delivery problem since Iíve owned it. The car acts like it runs out of gas after about 5-10 seconds at wide open throttle. As soon as I get out of the gas the power comes back. Interestingly, it doesnít act up unless the carís been run for about a half hour or more. Before that, it runs like a scalded dog!


Suspecting a fuel delivery problem, Iíve checked or replaced the following:
  • Dropped and inspected the stock gas tank. Inside looks new and no debris in the tank.
  • Inspected and blew out the pick-up line in the sending unit. Replaced the sock filter on the pick-up (old one was shot).
  • Replaced the hard line between the tank and engine compartment with a new 3/8 line.
  • Replaced the fuel pump with the correct 302/350 mechanical pump from Rickís.
  • Replaced the hard line between the fuel pump and carb.
  • Replaced the carburetor with a SMI Stage 2 Q-jet (has high-volume needle/seat and other performance goodies).
  • Replaced all rubber fuel hoses.
  • Replaced the fuel filter (old ones where never clogged).
  • Replaced the vented gas cap.
I made some of these changes while swapping engines or other performance upgrades. Since Iíve owned the car, Iíve used three different carburetors and a couple different stock fuel pumps on two or three different small-blocks. Iíve never been able to identify the cause of this problem. Iím tempted to install an electric fuel pump near the tank, but I have a hard time believing a fuel pump designed to feed a 302 with a big olí Holley carburetor canít deliver enough fuel for my car.

So, I could use some advice. Have I overlooked something?

69 RS, Hugger orange/black, Deluxe interior, Sony AM/FM/XM/CD, PS & PB. 355, forged pistons, Eagle rods, balanced, Iron Eagles, 9.4:1 CR, .038 quench, XR270HR Cam, Z28 intake, FAST-EZ EFI 2.0, Thorley Tri-Ys, TCI 700R4, 2400 stall, 3.42 posi
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post #2 of 12 (permalink) Old May 25th, 10, 02:26 PM
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Re: Need Help Diagnosing a Fuel Delivery Problem

Can you put a "T" fitting in series with the line feeding the carburetor. Then mount a fuel gage outside on your cowl panel. Then drive your car around and moniter the fuel pressure. Just do a temporary rubber line to it and wrap the gage in a towel and tyrap to the cowl panel or inside the cowl vent if you have a cowl hood.

Mike
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post #3 of 12 (permalink) Old May 25th, 10, 09:16 PM
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Re: Need Help Diagnosing a Fuel Delivery Problem

the big ole holley has 2 BIG ole float bowls. I know racers who forgot to turn the pumps on and made it to the 1/8 mile before noiseing over
You need an elect fuel pump

Larry AA/SA
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post #4 of 12 (permalink) Old May 26th, 10, 01:04 AM
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Brian
 
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Re: Need Help Diagnosing a Fuel Delivery Problem

Quote:
Originally Posted by larry A/SA View Post
You need an elect fuel pump
You've got a pretty mild SB, so I'd think the factory set-up would be able to get enough fuel to the carb. You're in the Texas heat. Is it vapor-locking? You might try a heat shield under the carb. It's cheap and easy, plus it'll keep the carb/intake charge cooler which equals more HP.

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post #5 of 12 (permalink) Old May 26th, 10, 01:49 AM
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Steiner
 
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Re: Need Help Diagnosing a Fuel Delivery Problem

Three carbs, two pumps, and everything replaced. Maybe it's not in the fuel system. Perhaps heat soak in an ignition component?

'69 Camaro
Dart 400-AFR 195-224/224 HR-Powerjection III TB with F.A.S.T. Sportsman XFI
TKO 600-Moser 3.42-Detroit Truetrac
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post #6 of 12 (permalink) Old May 26th, 10, 03:52 AM
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Re: Need Help Diagnosing a Fuel Delivery Problem

Quote:
Originally Posted by Steiner View Post
Three carbs, two pumps, and everything replaced. Maybe it's not in the fuel system. Perhaps heat soak in an ignition component?
good point or vapor locking could be an issue?
are any of your fuel lines near anything hot?
accel pump?

BILL D.
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post #7 of 12 (permalink) Old May 26th, 10, 10:24 AM
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Re: Need Help Diagnosing a Fuel Delivery Problem

After replacing all that it is obvious it ain't your fuel hardware. Heat, vacuum leak, ignition...
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post #8 of 12 (permalink) Old May 26th, 10, 12:49 PM
 
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Re: Need Help Diagnosing a Fuel Delivery Problem

Maybe timing isn't advancing. On my 67 RS the mechanical advance was sticking after about 4k. It would fall on its face after that. My mechanic figured this out by advancing it with his hand, and replaced the entire distributor for me.
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post #9 of 12 (permalink) Old May 26th, 10, 01:56 PM Thread Starter
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Mark
 
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Re: Need Help Diagnosing a Fuel Delivery Problem

Thanks everyone for your input and suggestions. Iíll try to answer everyoneís questions:

Vapor lock Ė The car had a square-bore Edelbrock (I forget the model) installed when I bought it, and that would get hot enough to boil the gas after a long drive. I put a 1/2 inch phenolic spacer under it, and that solved the boiling gas problem. It didnít solve the WOT issue. I had been running the 1/4 inch carb gasket recommended by SMI under the current Q-jet. I replaced that with a 1/4 inch phenolic spacer when I reinstalled the engine last month. It didnít seem like it made any difference.

Fuel lines near something hot Ė There is a header pipe about an inch away from the hard fuel line thatís bolted to the frame. I donít know if thatís close enough to be a concern. I was running stock exhaust manifolds and exhaust pipes on the previous engines, and they were several inches away from the fuel line. I was experiencing the same problem with the stock exhaust system.

Distributor or ignition problem Ė The car had a large-body Accel HEI distributor when I purchased the car. That was being fed about eight volts through the stock resistor wire. I rewired the ignition to get 12 volts to the distributor and figured that would solve the WOT problem. It didnít. Iím currently using a new Accel street billet magnetic pickup HEI distributor thatís fed 12 volts from the horn relay through a 40 amp relay (I got the idea for the upgrade on this site somewhere). Coil is a 45,000 volt Pertronix.

Monitor the fuel pressure Ė Iíve considered that before, and probably should. Iím using a stainless version of the 3/8 inch fuel line between the pump and carb, and donít want to cut it. I probably have enough spare fuel connectors lying around to rig up something to temporarily monitor the fuel pressure.


Here are a couple other possible causes that Iíve thought about:

Fuel tank not venting properly Ė Iím wondering if the gas cap may not be allowing enough air into the tank. Maybe after about a half hour the tank is under vacuum, and the pump is working against the vacuum to supply the engine. Iíve drilled a small hole in my old gas cap to try out that theory.

Tank pickup not oriented properly Ė Is it possible the tank pickup isnít oriented where it needs to be in order to pick up gas when the car is under heavy acceleration? Or maybe it doesnít go down into the tank far enough. I have had the pickup out of the tank before, but didnít pay much attention to how it was oriented or how far down the tank the pickup went.

Thanks again everyone for your suggestions. Keep them coming!!

69 RS, Hugger orange/black, Deluxe interior, Sony AM/FM/XM/CD, PS & PB. 355, forged pistons, Eagle rods, balanced, Iron Eagles, 9.4:1 CR, .038 quench, XR270HR Cam, Z28 intake, FAST-EZ EFI 2.0, Thorley Tri-Ys, TCI 700R4, 2400 stall, 3.42 posi
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post #10 of 12 (permalink) Old May 26th, 10, 03:03 PM
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Re: Need Help Diagnosing a Fuel Delivery Problem

[quote]Monitor the fuel pressure Ė Iíve considered that before, and probably should. Iím using a stainless version of the 3/8 inch fuel line between the pump and carb, and donít want to cut it. I probably have enough spare fuel connectors lying around to rig up something to temporarily monitor the fuel pressure.[quote]

Remove the SS line and make up a temporary rubber line with the "T" fitting. If you see the pressure going down on WOT then you have fuel problems.

Eliminate that problem first.

Mike
69 Camaro Prostreet - purple w/ghost flames , 502 w/ 871 blower


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post #11 of 12 (permalink) Old May 26th, 10, 04:38 PM
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John
 
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Re: Need Help Diagnosing a Fuel Delivery Problem

Tank neck sock,,it happened o me in my chevelle once i replaced it and it never happened again.
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post #12 of 12 (permalink) Old May 26th, 10, 09:21 PM
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Brian
 
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Re: Need Help Diagnosing a Fuel Delivery Problem

Another possibilty is the rubber lines that are in a few spots from the tank up to the pump are bent too tight and sucking shut when you get on it. Remote possibility, but it seems you've covered all the bases in the fuel delivery system if that's where the problem lies.

Experience...... It's what you get when you didn't get what you wanted.
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