Runs rich when warm

Hey yall, I just bought a '90 Cabriolet and am a bit puzzled. It starts great and runs great until it reaches operating temperature. Then it starts running really rich. Rich enough that it won't even rev up. Idleing it puffs black smoke and misses terribly. This is only my 2nd water cooled and after looking in my manual it looks as if it could be one of a number things wrong. Is there any one thing that goes crazy and causes this condition? It is bad enough that it can't be driven. Thanks much, Rick