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Old 07-07-2010, 01:18 PM
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Starved for fuel??

Just finished rebuilding my 1971 SS 396 Camaro (402 actually) with 9.5:1 pistons and 218/224 .510 lift cam. Found a correct numbers 1201 Quadrajet I had rebuilt and replaced the leaking fuel pump (bad diaphram) with a new stock pump (AC 40727). Car started right up and ran great. Now that I have 800 miles on it I took it out to see what it would do. 1st gear pulls strong to 5,000 RPM, shift into 2nd gear (M-22) and keeps pulling strong until at about 4,000 RPM it falls flat on its face due to fuel starvation. If I back of for a few seconds and get back on it it pulls to 5,000 RPM. I want to run a hose from the AC GF-432 fuel filter outlet (between the pump and the carb) to a container while running off a full carb. bowl to measure pump output, but I have no idea what output volume I'm looking for. Does anyone know how many GPM at a specific RPM the pump should pump? Thanks in advance for your help.
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Old 07-07-2010, 01:27 PM
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First off, did you pack the spring cavity of the pump with moly lube or anything else? if the pump is installed dry this causes the spring to overheat and fail. Small and big block chevy`s don`t have direct oiling to the fuel pump so this is why they must be lubed before hand. Without pre lube the spring gets hot and when it gets hot it gets weak to the point to where spring float sets in under high engine loads and upper rpm. You can check it with a fuel gauge and it maybe showing 4 psi, but that`s useless without knowing the volume. You can have as low and 2 psi as long as you have enough volume. In your case the Quadrajet (which I recommend) has a small fuel bowl working against you so the fuel volume is critical. Most of the time a high output fuel pump is needed. I`d get a carter, holley or edelbrock high output fuel pump and install it with a fuel pressure regulator, don`t use the small filter in the Quadrajet, use a large inline filter. This will take care of your fuel starvation issues.
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Old 07-07-2010, 08:21 PM
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Thanks for the input DoubleVision. Did lube the new pump. Was hoping to keep the exterior look of the engine stock. I'm a little surprised that the stock pump won't supply the required flow, I assume that it did in 1971. G.M. used the same pump all the way up to the 454. But then again, I have no idea what the required volume is. Any input as to the volume the stock pump is supposed to deliver would allow me to test the stock setup. Suspects, fuel tank filter sock, fuel pump (new but it's possible), in line AC filter?
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Old 07-07-2010, 08:49 PM
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The popular Carter GM6624 fuel pump puts out 120 gph or 2 gallons/min.

Your pump may not be the same volume, figure 60-80 gph as a WAG.
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Old 07-07-2010, 09:26 PM
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as for keeping the exterior of the motor looking stock i did that on my old jeep 327. just kept the old pump n line on there but ran an electric pump hidden inside the frame rail. looked stock and you'd never know it was there
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Old 07-07-2010, 10:56 PM
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I can surely understand wanting to keep it stock appearing. If the pump is a AC unit I`m surprised it`s having much of a issue. But, I`ve heard AC Delco went cheap some years ago. If your pump has a return line crimp it, and check the fuel pressure, it may go up over 5 psi and cause flooding. If not, test drive and do a wide open throttle blast. If it revs normally and doesn`t run out of fuel, you`ll know for sure the fuel pump can`t keep up with the demand. You could possibly look into the fuel tank sock, if it`s never been replaced it`s way past due and it could be collapsing under heavy pump draw.
A friend of mine has a 84 cutlass with a Vortec 350, RHS vortec heads, .215 duration roller cam with a Quadrajet, it too has a stock pump, when it gets to 4500 it does the same thing yours does.
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Old 07-08-2010, 05:04 AM
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Another possibility is the carb needle valve isn't large enough- there are several sizes above "stock" that can be used, and your BBC would be a definite candidate for a larger orifice than usual, IMO. Also the float level could be too low.
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Old 07-08-2010, 05:25 AM
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Cobalt327, the float is set as high as possible. I agree with the option of the inlet needle but, the carb was just rebuilt by the best rebuilder for carbs in Metro Detroit to factory specs, including the inlet needle. I am an engineer by training and like to make decisions based upon data. In this case I have none. My plan is to establish a "baseline" flow measurement before I make any changes. If the "baseline" is below your 60-80 GPH WAG (it's the only target I have) I will change the AC GF-432 inline filter (I have not replaced this in the 6 years that I have owned the car), then retake the flow measurement and look for improvement. If no improvement I'll change the pump and remeasure, if no improvement I'll change the carb inlet needle, if the problem persists it will be time to drop the tank. If that fails its time for a high volume pump.
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Old 07-08-2010, 05:52 AM
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Quote:
Originally Posted by scottyo
Cobalt327, the float is set as high as possible. I agree with the option of the inlet needle but, the carb was just rebuilt by the best rebuilder for carbs in Metro Detroit to factory specs, including the inlet needle. I am an engineer by training and like to make decisions based upon data. In this case I have none. My plan is to establish a "baseline" flow measurement before I make any changes. If the "baseline" is below your 60-80 GPH WAG (it's the only target I have) I will change the AC GF-432 inline filter (I have not replaced this in the 6 years that I have owned the car), then retake the flow measurement and look for improvement. If no improvement I'll change the pump and remeasure, if no improvement I'll change the carb inlet needle, if the problem persists it will be time to drop the tank. If that fails its time for a high volume pump.
Change that filter! lol

When you say the float is "is set as high as possible", I hope you mean within the specs given for it, as in 1/4", +/- 1/32" (just as an example), because it's easy to set it TOO high.

I looked around and didn't see any flow data for your AC pump, but I do have the specs from a '73 Chevy factory manual that calls for a pint of fuel in 30-45 seconds for the 454 BBC. That's a whopping 11.25 to 15 gph- BUT this is using the starter to turn the engine over (engine NOT running), so it's misleading in that respect. But safer, IMO.

Good luck.
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Old 07-08-2010, 06:20 AM
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Thanks Cobalt327. Already have the new filter. I like the idea of measuring the flow with just the starter motor. Now I have a good target. Thanks again!!
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