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Old 06-29-2005, 00:20   #1 (permalink)
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Fuel Pressure probs

Hey all,
So I have my new BG 650DP speed demon on and running now. The last two days have been raining non stop so I hadn't taken it anwhere to get tuned yet til today....
I installed at the same time a Russell fuel line with gauge. With only me tuning and it running bad I had 6psi+ up to 8 sometimes on the gauge.
now with my mate tuned it all up good, idles and runs smooth yet the gauge shows below 3psi pressure? this ain't good!

now the motor has been on a dyno with a holley 650dp and the AFR was fine to 6,000rpm, why would the BG make the pressure drop so much?

anyone got any ideas?

cheers
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Old 06-29-2005, 01:00   #2 (permalink)
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Re: Fuel Pressure probs

Just by changing the carby shouldnt make any difference to fuel pressure. Just try anonther guage. What sort of regulator do you run. I have seen holley regulaters do this. Just readjust it again while motor is running.
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Old 06-29-2005, 01:12   #3 (permalink)
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Re: Fuel Pressure probs

It shouldn't change - maybe mircoscopically.

You will still want 6'ish PSI on the gauge. I wouldn't be running it up to 6000 with 3psi - I would expect the fuel bowls to empty out at that low pressure and high rpm.

So, seeing that you've already done that and the AFR was fine.....hmmmm? This has me thinking your gauge isn't reading accurately. Does your gauge go to zero after the fuel pump is turned off? There may be some residual pressure that will take a little while to bleed down. Do you have another you can borrow and confirm pressure?

You running a return reg or single line?

What are your fuel levels in the bowls at idle? I'd guess they'd get quite low at high rpm with 3psi.

Did your mate touch the fuel reg screw? Unscrew your gauge and check for a blockage (sand/grit/old rubber hose/teflon tape).
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Old 06-29-2005, 01:17   #4 (permalink)
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Re: Fuel Pressure probs

there is no reg in the system as the pump is only mech.
havn't checked if its a zero with motor off yet, will latr on.

havn't pushed it hard with 3psi, but at idle the bowls sit a the levels they are set on, though the fuel jumps around alot.

thanks
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Old 06-29-2005, 01:43   #5 (permalink)
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Re: Fuel Pressure probs

OK, I though you had an electric pump

Being an rpm driven mechanical system, you'll find the pressure will go up as the revs increase, which will in turn decrease as fuel consumption goes up.

Rev the engine a little 2000 & 3000rpm and watch the gauge. It will hardly use any fuel (not being under load) and you should see the pressure go up. Did your mate change the idle rpm in the tune up/carby change?

Just as a comparison, my last stock mechanical pump (new) showed 8-9psi at idle at about 800-1000rpm.
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Old 06-29-2005, 02:46   #6 (permalink)
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Re: Fuel Pressure probs

the spring in your average mechanical pump gives you your pressure, it should remain constant regardless of rpm (unless your pump is too small to feed the carb). you should have a steady 6 psi at all times.

I'd be suspect of the gauge if the motor is running fine. You can use an air pressure gauge or something similar to check your fuel pressure gauge. fuel wont hurt them, i use a vdo turbo gauge to monitor my fuel pressure.

its unlikely your pump has died if the motor runs fine. the times i have had mechanical pumps die it is usually the one way valves that go, and you lose the top end performace of the pump first.
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Old 06-29-2005, 17:56   #7 (permalink)
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Re: Fuel Pressure probs

started it up this morning to take it back over to get a couple things looked at, I let it warm then turned it off, the gauge was sitting at about 7psi. hmm i thought.

I started it back up with bonnet open and it was still at 7psi :)

so its back to normal, any ideas what happened here?
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Old 06-30-2005, 00:36   #8 (permalink)
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Re: Fuel Pressure probs

Quote:
Originally Posted by pracy
started it up this morning to take it back over to get a couple things looked at, I let it warm then turned it off, the gauge was sitting at about 7psi. hmm i thought.

I started it back up with bonnet open and it was still at 7psi :)

so its back to normal, any ideas what happened here?
Probably a sticky gauge, small bit of foreign matter etc?

My interior gauge sometimes sticks when my electric pump comes on. I just turn the pump on and off about twice and it boots it into line.
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