Rapid fuel pressure fluctuation

[62Ranchero200]

Greetings Ford Six Fans,

As part of troubleshooting some carb issues, I installed a Mr. Gasket inline fuel pressure gauge into the flex line between my fuel pump and carb.

The fuel pressure fluctuates very rapidly between about 3.5 and 7.5 psi - so rapidly that it's almost impossible to read.

Does anyone else have a fuel pressure gauge, and if so does the pressure fluctuate like that?

If this is an issue, I'm thinking it's the stock mechanical fuel pump.

Thank you,
Bob the Builder

 

[rbohm]

8) it does sound fuel pump related.

 

[Frankenstang]

8) it does sound fuel pump related.

+1 I think anytime you're seeing 7-9psi fuel pressure on a carb'd setup it could be sufficient to push the needle off the seat (maybe affecting float setting as well at higher sustained psi) and enough to create a rich condition. I've planned on running some tests on a variety of mech pumps I have, but just have not had the time. I'm also planning on installing a 'quick fuel' brand regulator eventually (looks like a decent but cheaper holley knockoff), when I get the time to do some tinkering. A regulator is certainly a must with some mfgs like webers, but others have stated they're also a good idea (if not necessary) for proper performance on autolites, etc...might be something that would acutally have a quick payback ratio if it's affecting mpg's/performance sufficiently. Good luck

 

[thesameguy]

Any chance you've got 5psi and a needle that's vibrating with the engine? Is the gauge liquid filled?

 

[62Ranchero200]

Any chance you've got 5psi and a needle that's vibrating with the engine? Is the gauge liquid filled?

I'm not sure if the gauge is liquid-filled - I'll have to review the paperwork that came with it. It's an inexpensive Mr. Gasket, selected because it came with hose barbs on either side so it could easily be mounted inline. I thought of the vibration, and I held the gauge still while the engine is running - the fluctuation still continued.

Picked up a replacement fuel pump yesterday - haven't found any high performance mechanical pumps for the Falcon six, and I think a sound stock pump should be enough as long as I keep the single Autolite 2100. Plan to install today to see if that removes the fluctuation.

Thanks
Bob

 

[woodbutcher]

Hi Bob.If you have a liquid filled gauge you will be able to see the liquid through the gauge lens.
Good luck.Have fun.Be safe.
Leo

 

[First Fox]

My liquid filled under hood Fuel pressure gauge also fluctuates a bit (1 PSI either way) while idling, but smooths out as it is revved and steadies after it's above a fast idle. Have you ever checked for fuel flow with the fuel line off the carb and engine cranking? If not it may surprise you, but the fuel is delivered in spurts as the pump operates and certainly not in a steady flow.

I have always just adjusted my fuel pressure according to the pressure when the fluctuation stops. My weber for instance like 3.5 PSI. If I hold the revs at 1500 or 2000 and the gauge steadies at 4.5 PSI, I then lower it a tad and check it again at the "steady" pressure.

It is better in my opinion if the the fuel flow is choppy and the gauge jumps a bit at idle, but it needs to be properly set for cruise rpm and certainly for WOT. 8)

 

[Explorer]

Glycerin filled gauges are usually not cheap. While yes they do dampen vibration in general, they actually dampen the needle movement itself. Compared to a vacuum gauge that is very sensitive to change. If that makes any sense at all the way I described it.

 

[62Ranchero200]

My liquid filled under hood Fuel pressure gauge also fluctuates a bit (1 PSI either way) while idling, but smooths out as it is revved and steadies after it's above a fast idle. Have you ever checked for fuel flow with the fuel line off the carb and engine cranking? If not it may surprise you, but the fuel is delivered in spurts as the pump operates and certainly not in a steady flow.

I have always just adjusted my fuel pressure according to the pressure when the fluctuation stops. My weber for instance like 3.5 PSI. If I hold the revs at 1500 or 2000 and the gauge steadies at 4.5 PSI, I then lower it a tad and check it again at the "steady" pressure.

It is better in my opinion if the the fuel flow is choppy and the gauge jumps a bit at idle, but it needs to be properly set for cruise rpm and certainly for WOT. 8)

Apparently, it's not a liquid filled gauge.

Installed new replacement fuel pump. Needle still fluctuates, but in a smaller range: 5-7 psi at a fast idle. Should I be concerned about this? Could the needle valve in the carb be causing this?

Bob the Builder

 

[First Fox]

It's not a big deal Bob. Like I said mine has been that way on my daily driver for a million years. I believe it is just the pressure pulses from the pump. An electric pump will give a smooth delivery, but it is really not an issue.

As far as the needle and seat causing the fluctuation, No. No way. the needle doesn't open and close like that at all. The needle lifts and allows fuel in and just kind of stays cracked open a bit during operation. It doesn't open and close like a toilet float when you flush, but instead acts more like a toilet float when there is a small leak in the flapper valve and the commode just runs slowly. The needle and seat seeks an equilibrium and only really opens all the way when you are really draining the float bowl during WOT. The fluctuation you are seeing is regular pulsing from the mechanical pump. No worries.