carb - fuel pressure question

[Tetanus]

I've been having poor idling with my '75 K10 350 w/ a 4bbl. quadrajet. Someone recommended installing a fuel pressure gauge inline to see if there was a problem with the mechanical fuel pump (which is only a year or so old). The book spec says the fuel pressure should be 5 - 9 psi. My gauge cycles back & forth from about 2 - 7 psi. very quickly (approx. 10 times a second). Is this normal? I understand that the pump has to cycle because the pushrod pushes in and out making the pump function...but I'm not sure if it should be that noticable? I've never seen a gauge on anyone's carbed engine before so I don't know what to compare this to.

Anyone else have a similar setup?

 

[adamforsythe]

If I was you I would get a new pump...maby replace your pushrod that goes from the camshaft to the pump for G.P.........My pump was pushing almost 10 psi....thats way to much specs for a edelbrock is 5.5-6.5 I have mine set at 4psi because thats all it can handle.......Your carb is speced at like 3.5-6psi 9 psi is WAY to much you lucky your not running into the problem that I was having with fouling plugs.

I've been having poor idling with my '75 K10 350 w/ a 4bbl. quadrajet. Someone recommended installing a fuel pressure gauge inline to see if there was a problem with the mechanical fuel pump (which is only a year or so old). The book spec says the fuel pressure should be 5 - 9 psi. My gauge cycles back & forth from about 2 - 7 psi. very quickly (approx. 10 times a second). Is this normal? I understand that the pump has to cycle because the pushrod pushes in and out making the pump function...but I'm not sure if it should be that noticable? I've never seen a gauge on anyone's carbed engine before so I don't know what to compare this to.

Anyone else have a similar setup?

 

[Tetanus]

If I was you I would get a new pump...maby replace your pushrod that goes from the camshaft to the pump for G.P.........My pump was pushing almost 10 psi....thats way to much specs for a edelbrock is 5.5-6.5 I have mine set at 4psi because thats all it can handle.......Your carb is speced at like 3.5-6psi 9 psi is WAY to much you lucky your not running into the problem that I was having with fouling plugs.

Was your gauge steady @ almost 10 p.s.i., or did it cycle like I described mine doing?

 

[dyeager535]

I can't imagine those things can keep a steady pressure in the line. As you mention, it's a cam lobe, so it's no different in operation than an intake or exhaust valve.

If the float/needle/seat are ok, pressure really doesn't matter unless it's so excessive it blows the needle off the seat. All it's doing is filling the float bowl as it drains, which under no load is very little. If the float is set incorrectly (too low) it may be possible that the bowl drains under throttle.

I'd suspect ignition first for poor idle, if nothing else changed before the problem occurred.

 

[Tetanus]

I can't imagine those things can keep a steady pressure in the line. As you mention, it's a cam lobe, so it's no different in operation than an intake or exhaust valve.

If the float/needle/seat are ok, pressure really doesn't matter unless it's so excessive it blows the needle off the seat. All it's doing is filling the float bowl as it drains, which under no load is very little. If the float is set incorrectly (too low) it may be possible that the bowl drains under throttle.

I'd suspect ignition first for poor idle, if nothing else changed before the problem occurred.

Thanks dyeager535. You might have been right on the ball. I went ahead and replaced the intake manifold gaskets (suspected a small leak and already had the gaskets in the garage), vacuum lines, new PCV valve, distributor cap & rotor... and it runs better!

Before, I had to have the idle sped way up so the thing would idle. I couldn't adjust the mixture screws. (I believe the carb wasn't running in idle circuit, more like off-idle/part throttle...the screws did nothing) Now I can slow the idle back down where it's supposed to be and turning the screws has results!

One question, you mentioned excessive fuel pressure blowing the needle off the seat, how do you know if/when this is happening?

 

[dyeager535]

When this happened with mine, the float bowl overflowed into the air cleaner housing. You could feel the engine starting to "load up" as it got super rich, eventually the engine would stall. Popping the air cleaner off I could see the fuel that had "vented" out of the bowl.

 

[Kenny78]

With my edelbrock, you could see fuel coming out of alot of places it wasnt supposed to. This problem was only noticed right after I rebuilt the carb. Needless to say I was mad at my self and checked the off road needles and float height several times. I think it was so much more noticable then cuz of the new fuel filter!Anyways, the truck had a noticeable surge in the idle speed(RPM) and fouled plugs. I built an abortion of a fuel pressure regulator with a guage out of Mr. Gasket parts and fittings at Lowe's. I wanted to go slightly more compact, but the only comparably priced regs. lowest setting was about where I wanted to run. My pressure inline before the regulator was 8.5 to 9 psi. I was pleasently surprised that the dial on Mr. Gasket reg. is spot on. Instant consistent idle. Now I just have to clean/replace one month old plugs.

 

[Mike D]

Remove the gas cap. See if the idle and overall drivability improves. The fuel pump might be fighting a gas tank vacuum problem. The tank vent may be plugged up with mud or dirt-daubers ( flying insects ).