Carb, fuel pressure, 350 help

[mike@dw]

Hello all. Newly registered but have been reading here for a while. After looking for almost a year, finally picked up my first 40. It has a 350 conversion, 350 3spd auto trans, with a Holly carburetor.

I did not do the conversion and I know noting about carburetors (besides what I have been reading lately). I actually work for a fuel injection company and have never owned a vehicle with a carb.

The rig has run decent since I got it. I know buying a 40 that I did not build myself would take some going over. Spent two days going over everything. Replaced some gaskets, changed plugs, checked compression etc..


In the past couple days it has started dying on me when in gear or neutral. In park it idles OK.

There is a fuel pressure gauge before the carb and it is reading 12 psi while in park. Jumps around crazy from 10-15 psi while in gear. From what I have read fuel pressure on carb motors should be 6-8 psi and steady. I am assuming that the fuel pressure gauge I have is correct, so my 10+ psi is way too high and probably causing my issues. At least that is a place to start.

Since i am used to MPFI I feel out of my element here. There is no fuel pressure regulator on a carb system correct? I do not have an adjustable fuel pressure regulator. Should I look at replacing the pump or getting a fuel pressure regulator? Does the mechanical pump use any vacuum? I did not see a vacuum line on the pump.

Any input?

 

[inkpot]

Howdy! Most factory mechanical fuel pumps put out less than 10 psi, so I would guess that yours is aftermarket. I am running a Holly adjustable fuel regulator on my 350 with a Carter/Edlebrock. It is on the fenderwell between the pump and the carb. It is "T" shaped, so the fuel comes in the bottom, and goes to the carb from one side. The other side I added a liquid filled pressure gauge. On top is an adjuster screw with a jamb nut. I run about 8 psi on the freeway, but 4 or less on really bouncy trails. Your stalling problem sounds like you may just need to adjust up the idle screw on the carb linkage. Sound idle around 650-700 in gear, and 850 or so in neutral. John

 

[Downey]

You probably have a 15 psi pump. You could install a regulator to bring that down to 5-6 lbs. You could also have some other issues:
(a) Yes the fuel pressure can jump around between 5 and 15 lbs during driving and/or idling. Once the fuel pump fills the float bowl, the float causes the needle valve to shut off incoming fuel, consequently fuel pressure will rise until needle valve opens again. So this is always something that will fluctuate, kinda like standing by the door and constantly throwing your light switch on and off.
(b) You should adjust fuel pressure to 5-6 lbs stock, or 7-8 lbs if you install a spring loaded needle valve (one of the many Holley upgrades).
(c) You could have a blown power valve (common problem after many back-fires). This then would be the equivalency of gas flowing over the top of the dam, or through a hole in the dam, fuel simply never stops flowing even when your foot is not on the gas pedal. Engine runs rich, poor milage, and engine frequently dies at a red light/idle. Two ways to check for this:
(1) Pull air cleaner, look down throat of carb, looks blackish and wettish---probably a blown power valve.
(2) screw in air mixture screws on each side of the float bowl, one at a time. If you can screw all the way in clock-wise without engine fluttering or dying, then power valve is bad.

Hope these little tidbits help
Jim