Grey smoke is oil burning. If the oil is over it will smoke like that. Also, if the mower has been tipped on it's side for any reason oil will get into the cylinder and muffler. That will cause it to smoke for awhile, but it will stop smoking as the oil burns off. If neither of these are the case it's probably bad piston rings or badly worn valve guides. Both are repairable, but not always an economical choice.
Rick
May 2006
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