Truck in question: 1983 F150, 300-6 1V, np435, 4wd, single tank

Another day, another adventure!

Today I drove to see a friend about 15 miles from home. When I got close to our meeting place, I was accelerating towards 45 miles an hour and the truck started bucking, as if I'd taken my foot completely off the accelerator and put it back on.

I noted the issue, but it didn't repeat itself until I was leaving a couple hours later. Everything was fine for about 15 minutes until I got onto the highway. As I reached 30ish miles an hour, the same thing happened. The engine stopped accelerating. I let off the gas and slowly came back on and it seemed to fix it. It kept happening this time, so I babied it to around 45 and got off the next exit.

The truck lost power again as I approached a NAPA Auto Parts. I pulled in, and bought a new ignition computer specific to the truck, for $59. I figured that was my problem, as I've read about these boxes overheating and going bad before. I changed the box in the parking lot and it started right up.

Things seemed perfect for the next 20 minutes. As I got close to home, I lost power again, intermittent and briefly like before. The truck kept running, but it felt like I'd simply taken my foot off the accelerator. No strange noises or smells were present; all gauges read normal.


I'm thinking it might be fuel related. Just two days ago I drove from sea level to an altitude of 6500 feet, so I know my carb is running rich, but I didn't think it would cause a problem. I tuned my idle mix to be leaner, but did not change jets. I would expect more issues if it was flooding, though. I turned it off at Napa's and it turned back on totally fine. The fuel pump and distributor on this truck are at least a year old (Previous owner changed it), and I have recently changed plugs and wires.

Let me know if you want more information about the problem/truck/history.

 

Could it be a clog in a fuel filter? It sounds like you start to see issues during acceleration so it could be fuel starvation maybe?

 

That sounds like it would make sense.

But I just changed the carb mount fuel filter a week ago, and then drove across the country, so I don't think that would be a problem. Is there another fuel filter I could check farther up the line?

 

My truck has been tore into by everyone that's ever owned it so it is very far from stock. I have one that is attached to the carb itself, and another of the universal ones in the fuel line leading into the fuel pump. There is also a screen/filter on the pickup inside the tank, but I'd be really surprised if that was your issue here.

Sorry I can't be more help.

 

It's alright, thank you for your input! I appreciate anyone taking the time to help me solve my problems.

I wasn't even aware there is a second fuel filter. I'll check that and see if it has a problem.

It seemed to have absolutely no trouble idling or running around at low speed. I guess it only freaks out when the fuel starts flowing quick.

Also, when I replaced the electronic ignition box, I noticed the pull-apart connections had some kind of corrosion or old grease all over them. I scraped it off as best as I could with the tools I had on hand, but I will also try putting dielectric grease on them. I think the problem is more than just this, though, because it happens under load only.

 

You may not have one. Mine is one of the el-cheapo clear plastic ones just placed in line about 18 inches before the fuel pump.

 

Stock only has the 1 at the carb and the "sock" in the tank on the pickup tube.

I would change the filter at the carb so I knew it was good. More so if you don't know what shape the fuel tank is in.

Is your truck a feed back system?
Dave - - - -

 

It is not feedback, but there are a ton of vacuum lines,

 

when that happens to my '82 it is the manual fuel pump.
replace it and the problem should go away

 

I checked my battery voltage, 13.7 volts when running. So the alternator seems to be alright enough. I also cleaned the ignition box connectors. That should eliminate any obvious causes of no spark.

I went for a drive, and it took a long time before it had a noticeable lack of power. I got it to buck pretty good going up a hill near the end of the drive. I am gonna go get a new fuel pump and see if that fixes the problem.

 

Also check that horse shoe connection on the coil as I hear they can cause issues.
If it heats up and expands it may not have the needed connection.
I think there is another plug on that coil harness check it too.

Have you replaced all the rubber fuel lines yet? If not and you are going to change the pump pick up some new line and replace it.
You have a short piece at the pump from metal line to pump.
You should also have a short piece from tank(s) to metal line and if you have duel tanks a few short pieces at the switch valve.
They can look good but have a pin hole in them that will suck in air and not leak fuel.
They can also go bad inside and plug the hose.
Dave ----

 

Replaced the fuel pump 2 days ago, and the problem hasn't happened since.

If anyone else is planning to replace their fuel pump, make sure you put the camshaft-driven lever on top of the cam lobe, if your pump faces upwards. The smooth side of the lever rides on the camshaft.