I have a '78 F150, 351m, Motorcraft 2150 carburetor. After the truck dieseled a couple of weeks ago (blew gas back up through the carb on shutoff) it started to run rough after it had warmed up. I replaced the power valve (leaking diaphragm) but this didn't solve the problem. I'm also smelling a wierd almost-burnt fuel smell at idle. Any ideas? Startup is easy, it fires right up, but it begins to run rough after it reaches operating temp.

 

Could be running rich that could account for the weird smell. Check the automatic choke to be sure its opening all the way. Then check the float level.
What shape is your air filter in?

 

Float level is good, choke seems to be operating correctly. Honestly, I hadn't thought to check the air filter. Do you think it might be too dirty to allow good airflow after warmup?

 

I'd check it by removing the air filter element and let it warm up. See if it runs any different.

 

That's a cheap test, we'll give it a try tomorrow morning. My neighbor's are starting to squeak about working on the truck this late at night.

 

Vacuum lines all hooked up? Maybe it blew one out.
Jim

 

Vacuum lines are good, they're the first thing I checked.

 

When an engine diesels at the level you saw, it blows soot out of the carburetor because it's running backwards. The soot that came out of the throat of the carburetor also went through the carburetor's internal circuits, as the carburetor acts like an exhaust pipe in this situation.

I have had this happen before, and it went away after I drove it a couple times because it ran everything through. If you can't solve it through the simple checks that have been suggested so far, you'll need to go through the carburetor. Personally I would rebuild it. Before you do that, though, you need to solve the dieseling problem so that it doesn't happen again.