Thread: hesitation

Tweet

ok, I installed my new holley fuel pump and it runs great, but it's still hesitating at about 4500 rpm. But I did have the insight to put a clear inline fuel filter going into the carb. I took the internal filter out. When I start the car the canister fills right up. Then when I drive it hard and it starts sputtering and almost dying. After I get home I open the hood and there's barely any gas in it. Why would it fill instantly at start up but then become starved at higher rpms and not refill even after I slow it down. The more I'm seeing it, the more i'm thinking this is a fuel related problem I'm having with my car. Oh yeah it's a 1979 Chevy 350, stock HEI ignition, Quadrajet, aluminum intake off the original 305 H.O.

maybe the carb is the problem

that's what I thought too, but how would the carb keep the fuel filter from being full?

My best guess would be that at 4,500 rpm's, the valves are floating due to fatigued valve springs.

To isolate a fuel flow problem, first change all inline fuel filters from the tank forward and make sure there are no kinks in the hard lines. Change all rubber connection hoses, no matter how new they look from the outside. The rubber INSIDE the hoses could be separated from the fabric cord and causing a restriction. Next, unless you want even more grief in your life, replace the bronze filter in the Quadrajet with a new one.

Beg, buy, borrow or steal a MECHANICAL fuel pressure gauge and mount it temporarily on the cowl just in front of the windshield with duct tape and tie wraps or whatever. Using brass ferrules and copper line, run a fuel line from the gauge to the carburetor right where the supply line goes into the carb. A trip to the hardware store for brass fittings and a trip to the auto parts store for a couple of pieces of rubber fuel line will allow you to fab up a temporary connection to tee off the supply line. Now, make some runs down the highway and watch the pressure. You will need no more than 6 psi and no less than 3 psi. If it's more, install a pressure regulator between the pump and the carb. If it's less, use a different pump and larger fuel lines from the tank to the carb. I've seen Denny post on here that the minimum size for fuel lines should be 3/8" and I agree with him. Guys who are going to drag race usually change to 1/2" lines.

Good point Denny, thanks

how many fuel filters are there? I have the one in the carb out. and put the inline filter right before the carb. I'll replace the rubber hose from the metal line to the fuel pump

I have even had the gas cap loose and it's still done this

Posted by firebird:
"I'll replace the rubber hose from the metal line to the fuel pump"

You need to re-read my post. More carefully this time.

I read it. I'm hoping against the valve springs. They'll get replaced in due time, but for now I've spent about all I can spend right now. I checked all the metal lines for kinks. It looked good. the only rubber fuel line that's not new is the one between the metal line and the pump. I haven't checked the ones back at the tank. I looked at them today and they look like they have compressed fittings on them. I'll either have to order them from a dealership or start cutting and splicing. It wasn't doing this until I changed the fuel filter in the carb and adjusted the timing. I have a lot of posts concerning the conditions and how it occurs

I think I'm going to run the car low on gas and drop the tank and check the screen. won't cost much to do that. just don't really have a place to drop a tank. cops tend to frown on people doing that kind of stuff on the side of the road.

Originally posted by DennyW
I think that's why Tech was saying to use a pressure gauge where you could watch it. The sock in the tank could be gathering debre around it, causing a fuel blockage to the pump. Tech ?

Dang, another good point. I'm glad you're on here today to catch this stuff

i've rechecked it all honestly about 8-10 times. I used a purolator filter. I think before I goto the expense of setting up that fuel pressure gauge I'm going to fill my boat tank up sit it in the front seat with me. and run a line directly to my pump. If it doesn't do it anymore then I know it's between my pump and tank

I don't have that filter anymore. I took it out and replaced it with an inline clear plastic filter.

good thing I did that too or I'd be super pissed right now. Right now my absolute biggest concern is that there's excessive blow by exerting a counter force on the diaphram causing it not to pump properly. If that's the case then obscenities will flow like honey.

Originally posted by firebird45331
good thing I did that too or I'd be super pissed right now. Right now my absolute biggest concern is that there's excessive blow by exerting a counter force on the diaphram causing it not to pump properly. If that's the case then obscenities will flow like honey.

HUH?