There I was. Mowing merrily along today when, I got a big, white puff of smoke, and my power dropped very slightly and the engine ran just a little bit rough. Then, it cleared right up, and the engine resumed doing fine. For a few minutes. Then, it did this again several times before I put the old girl away. The engine was running smooth when I put the machine away. If I remember right, white smoke means it's oil. So, what could some causes be of this occurrence? How can dollops of oil intermittently get into the cylinders while the thing is running, without anything seemingly being wrong? What should I be checking?