I have a 1980 f150, that has a 351m with a 4 barrel Holley carb that was supposedly swapped from a 1979 f350. The other day I drove it about 20 miles and it ran fine. I parked it for about an hour and when I started it back up and drove off the engine seemed to stutter while accelerating causing the whole truck to buck. I could stop the stuttering/bucking by either letting off the gas and letting it idle, or if I accelerated hard the issue also seemed to go away. Effectively this issue is only showing up when gently accelerating or when I am trying to maintaining a constant speed, and it happens in all gears. Im assuming its a fuel issue and that the carb just needs to be rebuilt but I was wondering if there could be other causes I'm overlooking?

 

Probably not a fuel issue, since it runs ok when you step on it. You are using more fuel when you press the pedal closer to the floor.

I am going to guess you have a ignition problem. Either sparkplugs, wires, rotor, cap are a combination of these. The reason I say that, is in the mode when you are complaining, the engine is running leaner. This is a natural occurance, but a leaner mixture is harder to fire. If the ignition system is weak, then it can cause problems when the engine is in a lean cruise mode.

 

Could this be a restriction in the main circuit? At lower RPM, the idle circuit is in charge. At higher throttle settings, the enrichment circuit (power valve) kicks in.

 

Could be. I guess it comes down to what is easier and cheaper to try first. Pulling the top off the carb and taking the main jets out and checking them for being clogged should be pretty easy and cheap to check. It certainly could be the problem.

 

I've found with my 300 it does something similar if I don't let the engine warm up in the cold weather. I suspect my choke isn't quite tuned right.

 

One of the advantages of the Holley carb is that you can remove the fuel bowl and metering block without removing the carb from the engine.
remove and block the fuel line, then run the engine until it stalls.
Gaskets are available at most auto parts store.
You will be able to contaminates if there are any, the float level and the main jets.
Fuel would be my first suspect, contaminates or even poor quality.

 

I appreciate the advice from all of y'all! I was planning on pulling off the carburetor soon and doing a rebuild either way so I will start with that first. My thought is is that the reason for it having the stutter at a maintained speed but not during hard acceleration is that the issue is with the first two of the four barrels not pumping in enough fuel, but when the third and fourth barrel open up they let in enough fuel to allow the engine to run as it should (please correct me if this is flawed logic as this is the first carbureted vehicle I have really messed with). As well some things I should have mentioned in the first post and wich might help with diagnosis is the spark plugs and and spark plug leads to the distributor are about three months old as I recently replaced them. The fuel lines/filters from the fuel pump to the carburetor where replaced at the same time so they should not be clogged but I will re-check. At the time the issue arose it was not cold so I do not believe it to be a temperature based issue such as ice in the fuel lines/carb. As well the truck has a manual choke, I played around with it when the problem first started as I thought the truck was getting too much fuel, messing with the choke had no effect on the stutter whatsoever. And while I do not believe this to be related to the issue, it might be worth mentioning, the morning the issue started I had just filled up my tank with ethanol free gasoline, wich I had only done once before in the summer and truck ran great with it then. Again I appreciate all the help!

 

Please define "not cold". Ambient temperature from 30-50F is prime condition for carb icing. Do you have the stock air cleaner (with a snorkel drawing air from the grill) or an aftermarket open filter drawing air from under the hood? The stock air cleaner typically has provisions to keep the intake air out of the carb ice range.




Ah, another big clue. A bad batch of fuel is easy to eliminate as a possibility. Drain the tank and refill with fresh fuel from another vendor. Park in a bad neighborhood if you don't want to siphon the fuel out yourself. A specialty product like ethanol-free doesn't sell as fast as ordinary fuel, so it is very likely to have been sitting around a while before you purchased some.

 

I hope he didn't use over 10% ethanol fuel. His old truck is not setup for the higher ethanol content fuels they sell. Accidents do happen.

 

If it surges at steady speed look at the power valve.
If the vacuum needed to work the valve is not right it could be opening & closing.
Dave ----

 

To anserw further questions it was about 65 F when the issue started. And I do have an aftermarket intake that pulls from under the hood instead of from the grill. The fuel I put into it was ethanol free so a high ethanol percentage should not be the cause of the issue i'm experiencing. Once I get the carb rebuilt I will see if that has an effect, if not I will move to looking at the distributor and vacuum lines. Again I appreciate all the help!

 

Just an update to any intrested/who may be having a similar issue. I took my carb off and rebuilt it. I also replaced the vacuum line from the distributor diaphram to the carberator. After doing this the issue has resolved itself and the truck is running as it should. While the carb rebuild may have helped, I believe the real issue was in the vacuum line, I took the distributor cap off and sucked air through the tube and saw that it wasn't causing the rotor to move at all. Replaced the hose and it solved the problem. Probably could have fixed the issue without a carb rebuild but it needed to be done anyway.

 

Thanks to letting us know it is fixed.

To test that vacuum line thing, pull the line off the dist. and try plugging the line and see if it happens then unplug the line (like if the line had a leak) and test again.
I bet other than being a dog it will not buck like it did and the carb rebuild / cleaning is what fixed it.
Only because I would like to know with the above test and why we only make 1 change at a time LOL
Dave ----