What engne type? Briggs carb-on-tank? Change diaphragm.
If not, the symptoms are of a rich mixture. You checked the air filter - does it leak fuel? If so, and you have a float type carb - you may need to clean the inlet needle valve (inside float bowl).
phil_saunders@bigfoot.com
Phil Saunders
September 2008
Sounds like a 2 stoke engine and you adding too much oil to the petrol...
Could be a few other things but check your oil to petrol ratio