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Low fuel Pressure: The Sequel

Old 11-11-2007, 04:49 PM
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Default Low fuel Pressure: The Sequel

I posted a couple of weeks ago regarding low fuel pressure on my carb'ed 454 MAG Gen V. After MUCH painstaking troubleshooting I know what is causing the problem, but I am not sure what to do about it.

I added a Holley 850 marine carb and dart single plane manifold w/ 1" spacer, new Gil exhaust and MSD ignition. After the modification the engine starts up and runs great at idle. At anything above 1000 RPM the fuel pressure drops to nil and the engine eventually just runs out of gas. I tested everything from the anti-siphon valve to the fuel pump but the problem would sometimes magically go away only to reappear during my next session.

Well, to make a very long story shorter, it seems that fuel pressure is dependant on the vacuum presented at the fuel pump return vent: i.e. the little USGC vent tube that is supposed to dump fuel into the engine if the pump diaphragm ruptures. The problem would come and go because I would sometimes hookup a vacuum gauge to troubleshoot and would disconnect this line from the fuel pump and connect it to the gauge. At 2000 RPM I am showing about 23 (inches?) of vacuum. If no vacuum is applied, I have fuel pressure of about 9 – 11 PSI regardless of RPM, but if I apply any vacuum to that vent, the pressure drops off big time. I suspect this is kind of by design as it makes some sense that you would want more pressure at low vacuum (WOT). To verify I tried this with 2 new fuel pumps with the same results. I suspect that the stock carb “steps down” the vacuum presented to the pump return, but the Holley gives full manifold vacuum causing the pressure drop. Maybe this is common knowledge, if so I must have missed the memo. Hopefully this will save someone else a big (and expensive) problem – I was about to cut my cockpit deck to pull my fuel tank thinking some peeved ex must have dumped a pair of panties down my fuel fill.
Can anyone confirm this? Do most people just not hookup the return or its there some kind of step-down to reduce the vacuum signal to the fuel pump? Thanks.
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Old 11-11-2007, 06:15 PM
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Run the line to the air cleaner not the intake manifold where it is getting vacuum. Line is only there in case diaphram cracks on the fuel pump and the gas does not go in to the bilge when that happens. It is not intended to be hooked up to manifold vacuum. Should cure your problem.
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Old 11-11-2007, 06:39 PM
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Correct , the top side of the diaphram must br vented to allow it to go up and down and pump. Applying vacuum to it will hold it up and stop the pump action
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Old 11-11-2007, 11:43 PM
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That is only an overflow/dump line and it is not supposed to be hooked to any vacuum. It gets hooked to a nipple that goes through the flame arrestor. Its is in case the fuel pump diaphram ruptures. It will then dump the excess fuel into the carb instead of the bilge.

It should look like this, or you can drill a hole or notch the bottom of the flame arrestor and run the tube through there.
Attached Thumbnails Low fuel Pressure: The Sequel-525sc.1.jpg  
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Old 11-20-2007, 10:43 PM
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Default Follow Up

Griff
Thanks for the reply. What is the nipple off the arrestor? Is that part of the arrestor or add on? I have a Trick Marine FA with 3" height and cannot find a nipple that will work - I don't want to have any fittings inside the FA that could work loose. What's the best way to attach hose for fuel pump and also Valve cover breathers like you have?

Thanks
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Old 11-21-2007, 12:33 AM
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Do you have an SS mesh or K&N element????

Either way, you should be able to make the K&N stuff work to connect the hoses.
http://store.summitracing.com/egnsea...s=false&N=+115

CPPerf also has some.
http://www.cpperformance.com/product...stor-vents.htm

Last edited by Griff; 11-21-2007 at 12:42 AM.
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Old 11-21-2007, 12:41 AM
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Don't have K&N - I know they have a solution. It is SS mesh, not sure of mesh size, height is 3" Got it from Trick Marine.
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Old 11-21-2007, 12:46 AM
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Even mine on my factory merc were just screwed into the top of the arrestor.
I would just use the K&N stuff and then rivet or screw it.
http://store.summitracing.com/egnsea...s=false&N=+115


Even better from dana marine. I think Trick's arrestors were from them or gaffrig.
http://www.danamarineproducts.com/Gr...TOKEN=70409495

Last edited by Griff; 11-21-2007 at 11:17 AM.
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Old 11-21-2007, 12:49 AM
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Thanks for the link; I knew they had 2.75 inch but didn't know about the 6" - I can cut that down to fit my 3" high FA. I like it: clean and I dont 'need anything inside the cage where it could come loose and grind my motors.
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