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Old 04-01-2002, 02:33 PM
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Question Fuel Pressure Variance

The problem: Every time I turn on the ignition, I get a different fuel pressure to the carbs.
What I've done: The fuel system is virtually new. There's a new "sock" on the fuel pick-up tube in the tank,(the tank is clean & no water or trash floating around) I'm using a low pressure Carter 12V fuel pump. The line from the pump to the pressure reg is new. The fuel line goes in the bottom "in" of the Holly regulater. I have a fuel press. gauge on one side "out" and a fuel line to a dual carb block on the other "out". I've tried three different regulaters. The fuel line from the tank isn't anywhere near the exhaust, so I'm not developing a vapor-lock. The tank cap is properly vented (no vacuum in tank) I'm not useing teflon tape.
I can set the press. to 2 1/2 psig, run around a little and the pressure will creep up to 4-5 psig. If I reset the reg., the press. may or may not stay. It sometimes falls off to virtually nothing. The fuel lines go from the block on the fire-wall to the carbs which are on a Thickstun high rise so there's not enough heat there to be a problem. I keep going back to the reg. but it's so simple, (a ball-check, a pin under a diaphragm, a spring, and an adjustment screw.) I don't think it's the reg. but I can't come up with anything else. The entire system is clean. The one thing I am suspect of is a filter between the tank and the pump. The filter is new, and the pump manufacturer says if I bypass the filter I void the warrantee. Do any of you guys run a filter upstream of the pump? Would this cause the problem I described? If the filter was the problem, I could understand the press. falling, but it'll get higher more often than lower. What's up with this?? <img src="confused.gif" border="0">

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Old 04-01-2002, 04:15 PM
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I had the exact same problem with a similar setup to yours except it was a Mazda RX7 GT3 car I was running with a Holley 600. The problem it seems is that without somewhere for the fuel to go, the regulator has trouble regulating the pressure correctly. I don't have a proper explanation for it as I never disassembled the regulator or the Carter electric fuel pump which is supposed to have a built in regulator as they were both new.

My problem was trying to keep the fuel pressure below 5 psi, how I solved the problem was to plumb in a return line to the tank through the regulators second outlet and add a sort of brass pill into the line. In this pill I drilled a 1/16" hole as a restriction and the problem disappeared altogether. I can't explain why, maybe someone else can but it worked for me. I later added a Holley red pump to the same system because I was having a volume issue at top end and it worked fine also. I tried getting rid of the pill and return altogether and had the same problems with the red pump. I tried a smaller restriction in the hopes of solving the volume problem but it didn't help, it only made the pressure problem reappear.

I still don't understand why this worked, the guys at the speed shop weren't much help either. Seems like everybody just puts the regulators on and adjusts the idle pressure and forgets it. It was only under the pressures of road racing that this issue became a problem for me. The included literature with the regulator did not mention this nor did the pump. I found this fix by looking at top fuel dragster fuel systems and copying what they do. Later I found out that the best way to do this is put the regulator in the return line instead of using a pill, might work but I never tried it. I honestly don't know how that might work with this type of regulator or how you would plumb it.

Hope this helps, maybe some of the other members could shed some light where I could not. I would like to know also.
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Old 04-02-2002, 01:25 PM
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Thanks 4 Jaw, I'll tinker with a return line with a restriction like you described. Seems like putting the regulater in the return would give you full pump pressure at the carbs unless two regulaters were used. Maybe I'll put a small needle valve in the return instead of a fixed restriction, that way I may be able to accomodate two different carb configurations. I run two 2bbls for a while, then put on a 4 bbl for a while. On a flathead it only takes an hour to change out the whole thing.
Thanks again, I'm heading for the shop!
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