I cut my grass yesterday evening in my front yard. This evening I was going to cut my backyard. As soon as I started the lawn mower white smoke was pouring out of the muffler. I turned it off and did not see anything wrong at the engine. I restarted the mower and the smoke was still pouring out of the muffler and there a burning smell. After turning the mower off this time I notice black oil just below the muffler. My father just gave the lawn mower a tune up at the beginning of spring- he has always done the tune ups in the 6 years of me owning the lawn mower. He does live two hours away from me, so is unable to diagnose the issue over the telephone. Can anyone help with suggestions as to what could be going on?