Proper fuel pressure for BBC\Holley 770 - Chevelle Tech
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post #1 of 8 (permalink) Old Dec 10th, 06, 7:27 PM Thread Starter
 
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Location: Newcastle, Ont. Canada
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Proper fuel pressure for BBC\Holley 770

Hello everyone, long time reader first time poster. Been member of TC for about 3 years. I have learned very much through this site. Now I need some help determining the proper fuel pressure for my setup in my 70SS recreation. Engine is 402 4bolt main, Holley 770 Street Avenger carb with vac secondaries mounted on Eddie air gap intake. Cam is Comp Cam part #11-423-8 Intake dur 224, exhaust 230 @.050. Ignition is MSD 6AL with MSD Blaster 2 coil. Distributor is brand new with standard 20 deg advance. Base timing is 16deg BTDC and all timing is in around 3000 rpm. Problem is I smell raw fuel coming from the exhaust pipes. I checked my fuel pressure gauge at idle and it was reading 9.5 PSI. I think I have read here that Holley's like readings more in the range of 7.5 -8.0 PSI. The question is, would the higher PSI reading I'm getting cause raw fuel to go through the system unspent and what can I do to correct this problem. The carb was new back in the late spring and come to think of it I have never been able to get rid of a rich condition even after adjusting the carb with a vacuum gauge on full manifold vacuum. The jets are factory installed 72's in the front and 75's in the secondary with #65 PV and .025 nozzle. Vacuum at idle is around 13. Curb idle is 950 RPM.

I don't know exactly what fuel pump is on the car other than it's a Holley high flow. Perhaps someone can make a recomendation for a proper fuel pump.

Thanks
Gary
Newcastle, Ont.
Canada

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post #2 of 8 (permalink) Old Dec 10th, 06, 8:21 PM
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Ken
 
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Wink Re: Proper fuel pressure for BBC\Holley 770

I have a similiar setup.BBC 402, Crane cam, with 770 Holley Avenger. Performer RPM intake. I have the Holley electric Blue Fuel Pump 12-802 with a 12-803 fuel pressure regulator.The recommended pressure for the 770 Avenger carb is 5-7 psi.(from page 7 of the carb manual).
Adjust your pressure back to 6psi as a starting point. Find out what fuel pump you have and what regulator. Is your fuel pressure guage mounted inline after the regulator? The 9.5 pounds may be forcing the needles off the seats.This may be causing the raw fuel out the tailpipes problem.
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post #3 of 8 (permalink) Old Dec 10th, 06, 8:34 PM Thread Starter
 
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Re: Proper fuel pressure for BBC\Holley 770

Thanks for quick reply Ken.

My thoughts exactly. The pump is mechanical with no regulator. The gauge is mounted in-line. I think i will look at the pump and see if i can see a part # on it anywhere. Do you think it best to add a regulator or just replace the pump with a smaller PSI rating?

Thanks
Gary

The race is long and in the end it's only with yourself.<br /><br />Do something that scares you everyday.
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post #4 of 8 (permalink) Old Dec 11th, 06, 5:56 PM
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Ken
 
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Re: Proper fuel pressure for BBC\Holley 770

gary;
First try to find a number on the fuel pump to determine which Holley pump it is. Some of the high flow holley mechanical pumps require a fuel pressure regulator.Look on the Holley or Summit web site and it tells which ones require a regulator.I think a regulator would be the best way to go as you would have the ability to adjust it.
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post #5 of 8 (permalink) Old Dec 11th, 06, 10:05 PM Thread Starter
 
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Re: Proper fuel pressure for BBC\Holley 770

Thanks Ken,

I'll look at the car again after the holidays. I knew I could get great advice here. Thanks foor your help, I'll let you you know how I make out.

Gary

The race is long and in the end it's only with yourself.<br /><br />Do something that scares you everyday.
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post #6 of 8 (permalink) Old Dec 12th, 06, 10:02 AM
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Smile Re: Proper fuel pressure for BBC\Holley 770

It can be the fuel pressure being too high but it can also be the idle vacuum being too low due to perf cam,your sligthly retarded inital timing with the cam your running thus causing the power valve to open prematuly causing a very rich idle and low spped fuel mixture which is a very common thing to happen with a perf cam or it could simply be a bad/blown power valve.

That cam needs approx 18 deg btdc initital timing for best perf & idle vacuum so crank in a couple more deg initial with idle low like 550-600 rpms to keep mech adv from activating along with the vac adv unhooked/plugged when adj the initial timing.

Also,check the idle vacuum after you crank in the 2 deg additional initial timing to see what it is,it should be approx 13-14 inches + - with your cam.

Then check to ensure you have a 6.0-6.5 power valve if the idle vac is somewhere around what i est with the 18 deg initital timing and not the slightly retartded 16 deg initial your running that can cause sa lightly lower idle vac reading.

Scott

SCOTT
1969 CHEVELLE SS396,ORIGINAL #'S MATCH,GOT IN 1978,(In 2001 rblt/bored original 396 .030 to 402)/M20/12BOLT/3:31'S
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Last edited by SWHEATON; Dec 13th, 06 at 12:26 AM.
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post #7 of 8 (permalink) Old Dec 15th, 06, 8:20 PM Thread Starter
 
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Re: Proper fuel pressure for BBC\Holley 770

Thanks Scott,

Power valve is brand new as that was the first thing I thought of as well although I don't know why because I have never had a backfire through the carb. I dialed in the extra 2 deg making intial at 18 BTDC with better vacuum readings in the range of 13" but still have raw fuel smell at the tail pipes. unfortunately I don't have much time now to work on the car due to work obligations being in the Toy & Hobby wholesale business.

We did deterimine the Holley FP was the higher PSI output pump sow we will install a regulator shortly in the new year and I'm confident that will solve the problem.

I want to thank everyone for assisting me with this. I recomend this site to all the members in our car club. Chevelles.com rules!

Happy Holidays to all.

Gary

The race is long and in the end it's only with yourself.<br /><br />Do something that scares you everyday.
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post #8 of 8 (permalink) Old Dec 18th, 06, 9:34 PM Thread Starter
 
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Re: Proper fuel pressure for BBC\Holley 770

Update.

The fuel pump is a Holley 12-454-11 which should operate between 6-8 PSI.
After checking again yesterday it is clearly showing well over 9 PSI at 950 RPM idle speed. I know we could put on a regulator but I think it would be a better idea to replace the pump as I'm concerned it will continue to fail and eventually flood the engine. Any further thoughts? I only want to do this once.

Gary

The race is long and in the end it's only with yourself.<br /><br />Do something that scares you everyday.
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