As already mentioned black smoke means a rich mixture, too much fuel or not enough air, nothing to do with the sump oil (unless you have tipped the motor the wrong way and blocked up the air filter with oil).
Is the choke opening completely when off?
Someone may have used some wire to clean out the main jet, or the two holes in the bolt which holds the float bowl onto the carb, and enlarged the bores. The fuel inlet needle attached to the float might not be seating properly.
Just some thoughts, I have found those motors tend to be not too fussy with the fine details.
