Ktmktm01

Hello, I have a 77 F150 4x4 with a 351. During normal acceleration the engine runs well, but when I accelerate hard or drive up a hill, the engine hesitates/bucks and there is a loud poping noise under the bed right behind the passenger side. I’ve checked the spark plugs, wires, and distributor cap and they look good. Has anyone seen anything like this before? Thanks for the help.

Braggs

Sounds like you're running rich, missing on a cylinder, and fuel is getting into the exhaust. Heard this many times before.

Ktmktm01

Ok, I will see what I can do about that. Hopefully that’s the issue. Thanks a lot for the advice!

Ktmktm01

So I rebuilt the carb and adjusted it so it was more on the lean side, and it didn’t change anything. The truck still hesitates up-hill. Does anyone have any other ideas?

burnthelight88

Sounds like your loud popping is a backfire. Don't adjust the carb lean, adjust it correctly. During rebuild you should have done a dry then wet adjustment on the carb float. Then you should've installed it and tuned accordingly.

The normal driving going well and then hesitation on acceleration sounds like the carb's power valve is having an issue. Also check the elastometer valve (piece of rubber looks like an umbrella) that goes inside the accelerator pump of the carb.

Lastly, could be having a vacuum issue.

Ktmktm01

Ok, I will check that, then adjust it correctly.

Ktmktm01

I rechecked everything on the carb, and readjusted it correctly. That didn’t fix the problem. On a whim, I thought of something I did a while back right before I had this issue, but I forgot about it. When I bought the truck the cigarette lighter was unplugged. I plugged it in a few weeks ago, and not long after the truck started hesitating. I completely forgot I had plugged it in. When I unplugged it, the truck was no longer hesitating, in any condition! It’s so crazy, I thought I should tell people. There seems to be some sort of short or something from the lighter, or an electrical draw. I can’t think of anywhere that could be happening, but I’m just glad the problem has gone away. Not gonna reconnect the cigarette lighter... thanks for everyone’s help.