What does it mean when black smoke comes out of the muffler of your 6.75HP push lawn mower?

your fuel mixture is too rich it may be time to change your spark plug and/or air filter. a bad spark plug will not burn all the fuel correctly and a clogged air filter will not provide enough air for a good air/fuel mixture. Actually it depends on what color smoke is coming from it. Black smoke is fuel, however, blue smoke is oil, broken ring and such