First thing to do.
Pull the vac line from the AAP on the carb.
(driver's side towards the front)

Plug both ends.

I'll bet your fixed from there. Same thing on my 85 carb'd truck.
If you are in a colder climate, you'll want to replace the AAP before it gets cold again.

The AAP (Auxillary Air Pump) is specifically designed to add more fuel (richen the mixture) to smooth out the high idle when cold. Once warm, it's supposed to close. As it wears out, it continues adding fuel when it's not needed, therefore, making it run too rich, and that's the soot on you plugs.
Mine was intermittant. Ran like crap after warming up. If I shut it down for a few minutes, it would run good after starting after it was already warmed up.