Two possibilities - one - the engine has been tipped on it's side/front, and the muffler is full of oil - in which case it will burn off eventually ( check the oil level if this is the case), or TWO - the bore is worn - new engine required.
This assumes that it's not something silly like putting 2-stroke mix for the chainsaw in the mower.....(in which case don't worry about it - will do no harm at all except possible foul the plug)
phil_saunders(a)bigfoot.com
Phil Saunders
June 2011
Automatic Answer
While you wait for answers, we may have found some information which could help you.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lawn mowers.