I had the same problem (black smoke, rough running, stalling) on my 240 with a B230F engine. I had two different mechanics try and solve the problem, but fixes were temporary. Turned out to be the gas I was buying (Texaco). I switched to another brand, and the black smoke stopped and the engine runs smoothly.
Check the fuel pressure regulator, it may have gone bad and running the engine rich. Good luck
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smokes out tail pipe
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