ive got a mountfield sp470.i had the same problem as you,it was running very poor with black smoke.if you take the air filter off then undo the carb fixing screws and the 13mm bolt that holds the fuel tank on.remove the carb and blow all the holes out and the holes in the fuel tank where the carb sits.i used a drinking straw for this.the job is not as hard as you may think.my mower now runs great again.
rich
June 2007