Driving home from work yesterday the truck was working pretty good although it wanted to stall out at the first few lights. After that it was working great in stop-and-go traffic until I hit the highway (maybe 20 minutes later). I easily got up to 100kph/60mph but then it started bucking like a bronco (not a Bronco ) and wouldn't go any faster.

It stayed like that until I got off the highway (15 minutes). Then pulling away from a stop sign it had lots of power. Chirped the tires and got up to 50mph very quickly then started bucking again.

Brand new fuel pump installed 2 days ago.

Carb rebuilt a couple of months ago (maybe 500 miles on it).

New filter when the carb was rebuilt.

Where do I start? Replace the filter? Seems easiest.

 

Filter is cheapest / easiest change.

I don't know how confident you are in the carburetor rebuild. If you are sure it's right, and likely it is as attested by those short spurts where it runs fine ...
.... I'ld look at the fuel pump next.

I have a old Fuel Pressure / Vacuum Gage that I use to tee into the fuel line so I can run a long hose and secure it on the cowl but make sure hose is good and clamped as I do not want a leek. I used it more when all my stuff had carburetors, now it's only used on the '77 as everything else is FI.

Couple years ago when I went from Holley to Edelbrock carb on my '77 351M ..... I suddenly had a flooding at idle condition that I never had before. Carb was adjusted right. Put gage on it and took short drive ..... fuel pump was pumping 7 psi .... which is spec. So I added a Spectra (Mr Gasket same) Fuel Pressure Regulator and set it for 5.5 psi. Truck idled great ..... but when driving out the driveway, started sputtering up the hill. gage out on hood told me the fuel pressure was way way low then .... that POS Spectra FPR was now stragling my truck. Bought and installed a Holley #12-803 Fuel Pressure Regulator, adjusted using the gage, maintains 5.5 psi steady and truck runs great.

Point being ..... you need to check actual fuel pressure to be sure what's going on... or make guesses.

(nothing to do with your problem .... but as an explanation .... due to design of the float and length of arm Holleys will tolerate 7-8 psi but Edelbrocks will only tolerate about 6 psi max before fuel starts leaking past the inlet needle / seat .... Ford 351M / 400 fuel pumps will make 6-7 psi which is higher than many others .... it was to minimize chances of vapor lock)

 

Thanks, TBear. I'm pretty sure the carb rebuild is good. Took this picture a week or so ago - no problem at higher speed.

I'll swap out the filter.

 

Anyone else wanna chime in with thoughts or observations? Thanks.

 

Ever have the fuel pickup/sending unit out?
From the factory they had a lil sock type screen/filter dealio on the end of the pickup.
I normally just toss them and install a better filter in a more easily accessed place.

 

Who dialed in the carb and set it up initially?

When a fuel pump is replaced, the float setting and fuel level in the bowl should probably be checked all over again as well.

If it's bucking at highway speed and speed limited that sounds like it's starved for fuel, but it's tough to tell from here exactly what's happening.

 

500 miles on a carb and new fuel filter without issue, I'd be surprised if the problem was there. You mentioned the fuel pump is only 2 days old, why did you change the fuel pump?

Did you change any of the hoses (or routing of the hose from pump to carb)?
if YES, revert back to original hose / routing - you may be getting vapor lock.
if NO, I'd check (fuel pressure to carb)/replace the fuel pump.

 

The fuel gauge isn't working. I've tested the gauge itself and it's OK so the problem seems to be the sending unit. I will be dropping the tank at some time to fix this.

 

The old pump was spitting fuel onto the oil filter.

I changed to hose from the hard line to the pump - old one was really soft. No routing changes.

I'm going to throw a new filter on it and see if that helps. The old one is metal so not see through. New one is plastic/see through.

 

I set up the carb. It was running well. If other attempts to fix this don't work I will check the float.

Thanks.

 

Let us know please ..... and good hunting (for the problem)!

 

possible bad or going bad vacuum avdvance...for testing pull it off the carb, plug it and run it that way. It SHOULD run well without it. Hell mine runs better always off.

 

sorry for the double post...I meant"advance" and "pull the line off the carb and plug it"

 

I went through two rounds of fuel problems.

First time it was fuel starvation caused by old soft rubber lines getting sucked shut. When they dried out they got hard and after running for a while they got soft as cellophane and sucked shut.

Second time it was hot weather vapor lock aggravated by a 5/16" re-pop fuel pickup. She would run fine around town but open it up to run 60-70 MPH and she bucked.

"T ing" in a fuel pressure gauge as recommended above helped diagnose the second problem. I ran a 1/4" line to the gauge, tucked it under the windshield wiper and duct taped it to the outside of the windshield for the test. Drive around town and fuel pressure was OK. Stomp on it and it dropped to nothing.

An electric pump mounted in the rear where it is cool fixed it.

 

Appreciate all the help, guys.

I tried to install a new filter last evening but it was a bit longer than the old one and wouldn't fit between the hardline coming from the pump and the carb. (There is a plugged pipe from the deleted smog pump in the way.)

So I installed a new rubber fuel line to connect the carb to the hardline and moved the filter down to between the hardline coming from the tank and the pump. Unfortunately by that time the crappy little pinch type clamps were useless and fuel was leaking out around all of them. I will pick up screw type clamps after work today.

I did notice the short rubber line connecting the old filter to the hardline was pretty soft and may have been collapsing. I will update once I get back in there.