I bought a 78 f150 4x4 last fall and it had been sitting for about 2 years. When i bought it gas was pouring out of the carb so i rebiult it. I drove it daily for about 3 months and it ran fine. About 2 weeks ago after it would warm up i would drive it and get about 2-3 miles down the road and it would lose power, by the time i got to a spot where i could pull off of the road it went away and ran fine the rest of the way home. At first i thought maybe i just got something in the fuel and it worked itself out. I took the truck to work the next day and it ran fine. on my way home it did it again but this time it lasted a little longer but went away again.
I started trying to troubleshoot it and start looking everything over. Checked all the hoses for cracks. I changed the fuel filter and air filter and replaced the battery cables and made sure there was good contact. When i changed the filters i noticed the hot side of the electrical choke was off so i plugged that back in. I also checked the timing and it is good. Now it feels boggy all the time to the point that it is undriveable and does'nt feel like it wants to shift (automatic tranny) but idles fine, but if you push the gas pedal down it hesitates and backfires out of the carb. I know that it is a sign of running lean tried to richen it up alittle with the mixture screws but didnt help. So now i start replacing spark plugs and wires, cap and rotor, ignition coil, fuel pump, tighten down the bolts that hold the carb down , even a colder thermosta, and the pvc gets good suction. Not sure where to go next, i was thinkng maybe a new distributor and just breaking down and getting a new carb( but cant really afford it just to trouble shoot if it isnt the problem). Sorry for the length but please help me out here.

 

Welcome to FTE!
Double check the timing, I'm assuming it's a 351m or 400, and they are known for having timing chain issues. Also, they easily strip out the distributor drive gear when a high volume oil pump or fairly thick oil is used.

 

Ok, a couple of things caught my eye.

1st, bogging down and backfire sounds like a timing issue at first but....it sounds like to much fuel is left in the runners which gets to hot and burns back up through the carb.
2nd, it ran fine....then bogged....then ran fine.....repeat? Not timing. If it's jumping teeth you'll notice it. If it keeps jumping it's going to always jump the same way so it will get worse and stay there, not run fine later.
3rd, elec choke? Broken 'tang' on the internal spring? Can you disconnect it and run for a few days with a maual cable fed through the firewall? Something to ensure the choke stays open?
4th, is there a connection as to where this happens? Going up or down a hill? Can you check the fuel level in the bowl while on level ground?

Where are you located? Anyone in WA have a carb that TimeEater can borrow for testing.
I hope it's something simple!

 

I notice on thing, I don' t see where the ignition module was tested or replaced. This can also be one of those plugged up fuel intake socks also. A plugged fuel tank vent line can also do this. Put a tee in the fuel line and mount a fuel pressure gauge temporarily.

Put the proper thermostat back in. I think it is a 190-195°F unit. Make sure it is for a 335 series engine which is different than other engines.

 

Generally a over rich mixture will back fire though exhaust and a lean mixture will back fire through carb. Over advanded timing can cause carb back firing as can a worn cam or a bad valve(s) in head.

 

I did pull the top half of the carb and check the floats and they where fine. And sorry yes it is a 351m. Im going to get a fuel pressure gauge today and check it.

 

Yes, yes, yes. My carb ran rich all the time and I had a lot of backfire through the exhaust. Especially if I accelerated then let off the throttle, pop, pop.
It seemed if the engine was cold though, it went the other way, back up into the carb once in awhile.

 

Well I got a fuel pressure gauge today and and it was reading 10 psi, plenty, to much I do believe. A guy from work seggested to spray starter fluid or carb cleaner around the manifold so i did that and think i found the culprit in the middle on the right side was a couple of barely loose screws that when i went over it with the carb cleaner barely bogged down. So i tightened every bolt around the manifold and started it back up and rechecked the timing and it was a little off so reset the timing and readjusted the fuel air mix and bam runs great now. I'll wait till it cools down and try starting it again to see how she runs cold. Now am having shifting problems so ill make a new post in the tranny section. Thanks alot guys for the help.

 

Glad you got it figured out Look for tranny fluid in the vacuum line to the Modulator valve. It could have a bad diaphram and sucking fluid into the engine causing late or no shifting.

 

well the whole problem was an open vacuum line above the tranny that was from the back of the carb. It had popped off the modulator valve. Cant beleive it was something so simple, thats why it started running so rough. Oh well it was all stuff that I needed to replace anyways. Thanks again.