Fuel Pressure

[tyallis]

All,
Got something that i may just be paranoid about. Just got my 440 powered 66' Coronet put together into running/driving condition a couple week ago with very few issues so far (knock on wood). After a minor carb (4160 holley) float adjustment the other day, I noticed that my fuel pressure is holding LONG after i kill the engine (9 lbs), and is running higher pressure than normal at idle (10+). It will eventually return to 0 lbs after sitting over night; I assume it's slowly trickling by a check valve in the mechanical fuel pump. My gauge was reading 4-6 lbs until recently. Is this any reason for concern? Thanks in advance.

 

[PurpleBeeper]

Wow. On a carbureted engine you're looking for abut 5psi with a max of about 6-1/2 psi and ONLY that high if your fuel pressure drops at the top end of the drag strip (e.g. dual quads with a 10sec. car). 5psi is optimum from what I know. Higher fuel pressure can overcome the needle/seat & float and flood the carburetors.

I'd "guess" that your fuel pressure drops off because it's trickling down the carburetor throat. You have a weird problem. Maybe other more knowledgeable fuel guys will jump in.

 

[coloradodave]

Try a different gauge...

 

[tyallis]

The fact that it's holding pressure hours after the engine is off is also goofy behavior, correct? At first I thought the gauge was stuck; so I loosened a fuel fitting... fuel sprayed everywhere and the gauge dropped to 0. Hmmmm??

 

[PurpleBeeper]

The fact that it's holding pressure hours after the engine is off is also goofy behavior, correct? At first I thought the gauge was stuck; so I loosened a fuel fitting... fuel sprayed everywhere and the gauge dropped to 0. Hmmmm??

I'm running electric pumps & didn't have a fuel pressure gage on mine when I ran a mechanical pump. Mine certainly bleeds off pressure down to zero in about 1 minute after I shut off the pumps. "Maybe" your needle & seat is holding the pressure unusually well? Maybe your gage has gone bad & reading higher than what you really have? Do you have another fuel gage you could swap in to check?

 

[oldbee]

I think the first question oughta be what brand of mechanical pump are you running? & maybe change it out.

 

[62 Dart Convertible]

Not unusual for fuel line between mech pump and carb to hold pressure when engine turned off. Pressure will bleed off by either/both leakage past non-return valve in pump back to tank and past needles and seats in carb as floats drop due to evap of fuel in bowls.

The high pressure are unusual - 9psi is a little high for the carb. As already said, try another gauge to confirm. What fuel pump are you running? If high pressure is causing problems you could add a regulator or change pump. Since pressure is holding when you shut it off, it seems your carb does not have a problem with the pressure...

 

[tyallis]

Thanks for all the words of wisdom. I will toss in a different gauge to confirm issue. I'm running a Carter "street super" #6903 mech. pump which is supposed to supply btwn 6 and 7.5 psi. It is new (along with just about everything else under the hood, including the $30+ Earls fuel gauge).

 

[tyallis]

Changed gauges, resulting in the same behavior. Got a new pump on its way... guess we'll give that a shot.

 

[PurpleBeeper]

Changed gauges, resulting in the same behavior. Got a new pump on its way... guess we'll give that a shot.

Well, 62Dart made a good point.... you could just add a fuel pressure regulator like they use on electric pumps too. Oops! I re-read your post that it USED to stay at 4-6psi.....yeah, your fuel pump is getting funky