The Stovebolt Site Forums The Shop Area Engine Shop 350 with carb problem?

I have a 350sb with an edelbrock 4bbl performance carb. I've had this setup for almost 2yrs. Recently at around 50mph the truck feels like it's running out of gas but it surges right back and this repeats over and over again until I get to over 60mph. Any suggestions?

What you're experiencing is called a "surge", and it is actually running out of gas- - - -almost. The fuel filter is probably clogged, allowing the float chamber to almost empty before the fuel pump can refill it. If the filter is new and clean and the problem persists, there's either a problem with the fuel pump, or the line from the tank to the pump is restricted. Backtrack one part at a time from the carb to the tank until you find the clog.
Jerry

Wow. Thanks for the quick reply. I just came back from buying a new fuel filter. I'm also going to pull a spark plug and see what color it's got. I'll let you know how it all works out.

I replaced the fuel filter and took her out to the interstate and ran her up Monteagle Mt. I tried everything I could think of to recreate that "surge" but it ran great.
Thanks Hotrod.

Good show! I've got a good friend who used to race SCCA sports cars with me who lives near Raccoon Mountain.
Jerry

Bad news. Took a run over to Chattanooga today and had the same "surging" problem three different times. Could a fuel pump work okay sometimes and not others? I always thought that when a fuel pump went it just stopped working altogether. Thoughts or suggestions?

It's possible there's a restriction in the fuel tank pickup screen, or maybe the flex line from the frame to the fuel pump is collapsing, restricting the fuel flow to the pump. My 47 Mercury would stop completely after a couple of miles, and restart in 15 minutes or so. I finally found a big grasshopper lodged in the fuel line inside the tank.
Jerry

for the sake of simplicity, I should start at the pump and work my way back to the tank?

Be sure the flex line is in good condition first, or just swap it out for a new piece of fuel line. They can weather-crack and suck air without leaking fuel to the outside. A piece of new hose won't break the bank. There's probably another flex line where the frame-mounted steel line meets the tank. I like to use a vacuum gauge to check for good fuel pump operation, and seep leaks in flex lines. Disconnect the fuel line, plug in the gauge, and idle the engine. It should run for a while on the gas in the carburetor bowl. Back at the tank outlet, there should still be a good vacuum with the engine running, 12 inches of mercury or more. If you've got good vacuum that stays put for a few seconds after the engine stops, look inside the tank for problems.
Jerry

Willy, If this Edelbrock is an AFB design look carefully at the carb top near the float valve area to see if there is a brass hex plug, 3/4 hex if I remember correctly.On some of the AFB and etc there is a fine screen under the plug which can blind and cause fuel starvation.probably one for primary and another for secondary.
Study Jerrys suggestions and interest in the line before the pump carefully.Small leaks here can be tough to suspect but really cause trouble.I have proven leaks here by directing the fuel flow after the pump onto a container and look for air entrained in the flow whth the engine running on what was in the carb.Many times they won't leak with the engine shut down.

My plan was to do all the things Jerry suggested and I was going to put a new fuel pump and new fuel hoses from Advanced Auto. There is a good chance that pump is orginal to the 1980 c10 the engine came from. Not sure about the afb carb designation but I will check that out. I had pulled the intake fuel line at the carb to see if there was a filter in there but there was none.

Reading again more carefully, and I see the surge going away above 50mph.If the surging goes away when you open the throttle more it is hard to think insufficient fuel delivery--especially if you can open the throttle and run it up a lot or step into it when going uphill without running out of fuel, which will be obvious--you'll have to let up on the throttle to keep it running well.
The part throttle/light load circuit on these Edelbrock/Carter AFB is managed by the metering rods controlled by the attached piston with a light spring under it.At high vacuum the piston/rod is pulled down into the main jet leaning the mixture until throttle opening lowers the vacuum allowing the spring to raise the rod into a place of less effect on the jet size--richening the mixture.
Edelbrock has a tuning kit of several spring choices(different strength springs)and several rod sizes.This kit allows easy(?) changing of the mid range mixture without dismantling the carb.The power piston/metering rod/spring assembly jumps up to look at you when the little plate with one screw is removed.Two plates-one on each side of the primary(front)throats.
You say you have had this carb 2 years.During your ownership has this condition always been there or has there been a change in it's habits?
Also we have been experiencing an extended hot season which effects underhood temp and our beloved tubing headers don't help a bit.The AFB (aluminum four barrel)is a great heat conductor to the fuel it's handling and gasoline doesn't like to be heated much before being mixed with air in the carb.If sensitivity to heating is suspected an insulator or stack of gaskets under the carb may help and attention to fuel line routing can help.
In general, keep the engine /fuel/carb/incoming air/engine compartment cool.

Good analysis...and advice. I'm curious about how long this has been happening, also.

cm

Willy, You might prove or disprove this temp related theory by planned and methodical testing in the cool part of the day compared with the hotter time of day.But you were already thinking that.

This forum is so valuable to guys like me, to a point. Then you're way over my head. LOL. However, I'm reasonably sure it's not heat related. Sometimes it happens within the first 15 minutes or so of driving and sometimes it's after an hour or so. When I said it happens around 50mph and goes away around 60mph the speed increase is from the "surging" and not simply by giving it more gas. Hopefully I'll be able to pull the pump today and change out the fuel lines and get back to you with the results by tomorrow.
Wrench, I was at a car show this past weekend in Burns. Camped out in Montgomery Bell State park. If you were there you might remember my German Shepherd, Willy, I had with me.

Willy, When the problem occours at say 50,if you continue to open it up does the surge go away or does the engine refuse to go faster which it would do if fuel starvation were the problem.
Normally when you try to accelerate beyond the 50 the vacuum would drop and the metering rods would retract calling for more fuel--so if the fuel was there(in the bowl) the mixture would richen-power and speed would increase--but if the fuel level were low because of a deficient delivery(causing the surge), the engine would die and backfire or miss severely because of the increased air/insufficient fuel.
This test will tell you if the fuel delivery is deficient.

Wish could have gone to the car show--I drove by 6 times as I live 1 1/2 miles beyond there.Sometimes just not enough saturday to get er all done.

just an update: it was a real bear getting the pump out. The way the engine is squeezed in I had to pull the alternator but to pull the alternator I had to remove the overflow tank and the alternator mounting bracket. As hard, and hot, as it was I'm dreading putting everything back more then I did pulling everything out.

updating the update: Got the new fuel pump and hoses in and put everything back together. Runs great - but still surges. Looks like the gas tank is next. The tank is only a 2 year old stainless tank but "stuff happens" and gets caught in the filter I guess.