I have a Craftsman push mower with a Briggs engine. The engine use to be easy to start, always on the first try. Lately it hasn't been so easy and sometimes I can feel it pull back when I pull it. Then the other day when I was using it, it started smoking terribly. White smoke. After I stopped it I noticed motor oil leaking from the exhaust. Any ideas? The mower has not been tilted at any great angle.