Worn cylinder and high oil level. I can set my watch by my 816S doing the same thing. If I top the oil to Full and then crank and warm up for about 3-4 minutes at mid throttle and then go to mowing.. it will do it almost every time. 30-45 seconds and it clears up, eats the oil, and we keep going. I have a new block for mine just haven't gotten around to building it yet. Pull the air filter and you'll more than likely see the oil in there. It is pushing it through the breather by the valves, into the intake, and eating it. The thing about it is, as worn as the cylinder is in my block, it still makes good power. It is no spring chicken, but it pulls the 50" deck...