The 300 in my 81 has recently started to idle really rough, at red lights and at stops I have to keep the rpm's up to prevent stalling. I think this started when I changed the air and fuel filters. I installed a cheap Delco inline filter. It's putting out a white/gray smoke which doesn't make me think its running rich but it acts like its starving for fuel. After letting it run a few mins I notice the muffler is super hot. I didn't remove and vacuum lines or anything and the tank is on full. What should I check next?
I'm thinking try to adjust the carter 1bbl, verify cat isn't clogged, and change/inspect spark plugs.

 

A sudden change like that, I suspect electrical and would want to have the ignition module checked (parts stores can sometimes do this).

Is there any difference between when it's cold (sat overnight) and warmed up?

 

I just replaced the voltage reg a few weeks ago and the ign module is about a year old so I'm not gonna rule it out, didn't even think about that. Since I changed the filters the truck actually runs good on startup and bad when warmed it seems. Before now, I'd have to idle it up a minute on cold start and then it ran great after that. I suspect air/fuel to be te culprit but haven't had time to check the plugs and don't have an air fuel guage to check for stochiometric. If its getting too much fuel due to the new free flowing filter I wonder if an open element air cleaner would help?

 

Well, the usual thing is to un-do your recent change; in this case, put your old fuel filter back and see if the problem goes away, most failures (or changes in behavior) are the result of something having been changed.

It is possible you dislodged some dirt or crud and allowed it to enter the carburetor, but I for some reason am still focusing on electrical, so... I'd go double check and maybe clean/reset the electrical connections in the same general area as where you were working, maybe do something called a wiggle test where you wiggle wires/harnesses while the engine is running and see if the behavior changes.

The muffler getting super-hot is telling me combustion is happening inside the exhaust system instead of in the engine like it should be, but unless you can say for certain it didn't do that before your filter change (and is therefore also a new symptom) it might have to treat that as a different, unrelated symptom, possibly pointing to the same cause, possibly not.

If it's running rich and you think a new air cleaner might help (I really doubt this) just run it for a bit without the air cleaner.

 

Yes that's true I've never touched the muffler while running prior to this. And also I can't rule out a head gasket because it did this at the end of a 150 mile journey and I haven't checked the oil yet. The smoke looks kinda like oil getting past the ring smoke but the truck has never done this and the engine has maybe 60k on it, it's a jasper equivalent reman with the original carb

 

The carb doesn't use hardly any fuel at idle. So it's probably not a fuel starvation problem, that would rear it's head when you had it on the road with a pull on it. That's when you use a lot of fuel.

But, Chris mentioned dirt being stirred up when the filter was changed. That could be a problem that could certainly happen. If you want to check this, take the air cleaner off, get a ladder and a flashlight, and get setup so you can look down the carb throat with the engine idling. Go start the truck and get it so it will run by itself. Then run out and look down the carb throat while it's idling. If you see any fuel at all going down into the engine, you have a problem. You should see no drips or any liquid fuel going down into the engine. The only time you will see this is if you move the throttle while looking.

If you do see fuel dripping into the engine, get a plastic handled screwdriver and grab the metal shank and use the plastic to peck on top of the carb. Do this a couple of times and see if the engine clears up and the drips go away(if you have them).

 

Alright I will try that. Garbage/rust in the fuel is very possible as the truck has the original tank and this was the first time I filled it up to the neck since I've owned it.

 

When it first started this last week I ran it WOT and it seemed to make it idle much better until I cranked it this week. Truck runs good under full throttle it seems

 

Changed the plugs today. They were ngk and were super carbon'd up. Pure black no gold or bronze at all. Put some autolite copper plugs in it and now gotta find why it's getting so much extra fuel when I get time.

 

Did you peck on the carb yet and check for fuel going down in the carb? That's probably where all the fuel is coming from.

 

Not yet I broke wire #6 so I haven't run it again after the plugs. On the back of the carb there is a circular dial type piece I guess for the choke? It turns freely both directions and doesn't move anything in the carb if this is my problem.

 

I think you have found what very well could be the problem area...

 

Read this and see if it helps...

https://www.ford-trucks.com/forums/1...ent-101-a.html

 

Read up on the yf carb, thanks for the link. I looked down into the center while running and can definitely see fuel dropping pretty heavily. Tried tapping it but saw no change, the truck will barely stay running long enough for me to go from the cab to the carb. Replaced the plugs and it ran great for a few mins but I'm sure they are fouled again

 

Your fuel level is too high, you must have dirt sticking the needle open in the carb. If pecking on the carb doesn't work, you will have to take it apart.

P.S. Something else that works well; If you have a rubber line somewhere between the fuel pump and the carb, you can get the engine running, and pinch the line shut. You will find the engine starts running really good as the fuel level starts dropping in the carb since you have the line pinched. Keep pinching it till it starts to run rough and tries to quit. As soon as that happens, un-pinch the fuel line. The fuel will rush into the carb past the needle, and sometimes this surge will push the dirt out of the needle/seat area and cure it.