I have a Toro 16 HP Lawnmower, B&S engine. It runs good for about 20 minutes or so and then it starts backfiring and dies out with a puff of black smoke . I cleaned the carburator, replaced the oil, fuel and oil filters and plugs. Again it ran great for 15 - 20 minutes and then backfires and dies out. If you leave it for a while, it starts again but dies out some minutes later when it is hot. Same thing over and over again. What would cause this?