Went out this morning to see if I could figure if the carb float was the problem. Checked the oil and no gas contamination was found. Decided to try and start it before I tore into carb. It started as before, running rough and emitting black smoke, but it kept running. It seemed that at least some of that smoke was being forced back from muffler instead of going straight out. After about a minute and before I could examine the muffler more closely, she suddenly starts running normally, black smoke is gone and all is good with the world:laughing:. Could it be that the exhaust was somehow obstructed, perhaps by a dirt dobber nest or something? Don't know, but thanks for the reply. Gotta go mow...:thumbsup: