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 Correct way to set fuel pressure with Mech Pump 
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Joined: Tue May 06, 2014 6:17 pm
Posts: 4
Maybe a dumb question hope this makes sense ....... BBC, using a mechanical HO fuel pump and regulator to keep max pressure 6.5-7.0 Reg has a 1/4"pee line return to help with vapor lok set at 6.5 idle when I recheck after a week or so wot pressure around 10psi !!!!

Engine runs fine no loss of FP at WOT etc but have had an issue with floats going high assume from excess FP gas in the oil

Understand the issue with guage getting hot etc vibration etc not the guage I use same test gage on an electric pump engine no issues
Does the mech FP pulse at idle speed making pressure bounce around ?? should I set at 1500-2000 rpm then should the fuel pump pressure be more constant ?? to get an accurate reading


Tue Feb 16, 2016 5:26 pm
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Joined: Fri Feb 08, 2013 5:50 pm
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What regulator?


Tue Feb 16, 2016 7:18 pm
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Joined: Tue May 06, 2014 6:17 pm
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Aeromotive seems very sensitive


Tue Feb 16, 2016 8:55 pm
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Joined: Thu Mar 28, 2013 6:09 am
Posts: 726
I have seen a bouncing fuel pressure gauge do that with a mech pump when the pump spring is broken..


Wed Feb 17, 2016 7:56 am
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Joined: Tue May 06, 2014 6:17 pm
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barryh wrote:
I have seen a bouncing fuel pressure gauge do that with a mech pump when the pump spring is broken..



Yes I have checked the pump its all good ...no problem making good fuel pressure at end of stripe just sometimes too much if I don't set reg correctly ......

In the end maybe I overthinking this ... any reason not to set pressure at 1500 rpm @6.5 with my pee line disconnected and port plugged ?? does that make sense ?


Wed Feb 17, 2016 12:03 pm
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Joined: Mon Dec 29, 2014 1:26 am
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A diaphragm style mechanical pump (spur or gearotor style are positive displacement and different animals altogether) should not increase pressure with increased RPM. Cam lobe pushes against the spring for the intake stroke, spring pushes the fuel out once the cam lobe moves away. That means that if there is no output flow - the N&S is closed or very nearly closed, deadhead regulator is seated - the spring cannot push the fuel out, that means the next time the cam lobe comes around for another intake stroke it does nothing, since the diaphragm never moved from the last time.

Higher RPM just means more intake strokes and you should only see a bump in pressure if the diaphragm has fully extended since the last intake stroke, like when free-flowing with the line disconnected. If you see pressure rise with RPM, something in the pump is broken.

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Thu Feb 18, 2016 6:47 am
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Joined: Mon Dec 29, 2014 1:26 am
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Is the regulator mounted on the engine? If so and you can't move it somewhere else, rotate it 90* (if the diaphragm is horizontal, turn it vertical). Engine vibration can sometimes move the diaphragm like a piston.

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Thu Feb 18, 2016 6:53 am
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Joined: Tue May 06, 2014 6:17 pm
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cat herder wrote:
Is the regulator mounted on the engine? If so and you can't move it somewhere else, rotate it 90* (if the diaphragm is horizontal, turn it vertical). Engine vibration can sometimes move the diaphragm like a piston.



Hmm interesting .............yes reg bolted to front of head diaphragm is vertical but you bring up a good point re vibration I will try to relocate or isolate to reduce vibration and see if that changes anything ( engine does not move much has torque strap ) but you never know ...thanks for the reply


Iam I correct in disconnecting the return vapor line and plugging port when setting fuel pressure ?


Thu Feb 18, 2016 10:08 am
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