I recently bought an '83 240 DL with that had a rough idle. I removed the air intake manifold and cleaned the throttle body -- there were minor deposits but if around the throttle plate that cleaned easily with carberator fluid (use a rag - don't let fluid get into throttle position switch). This solved the rough idle - now idles very quitely.
The black exhaust does sounds like a bad AMM. Had that problem with my '85 240 -- bought a used unit from ebay -- problem solved.
Good luck.