I would 1st. empty the "bad" fuel mixture out and then pull the spark plug, and clean it with emory clothe or fine sand paper (without changing the gapping!)then clean carbon and metal dust off, and then check out the air filter and make sure it is clean and also while you have that open make sure the carb is working properly (opening and closing). I just had the same experience with my roto tiller and it is working great now! :) Good luck!
Matthew
June 2008