This engine is on a lawn tractor and suddenly it starts fine, runs for anywhere from 3 to 5 minutes and then suddenly shuts down with a huge cloud of smoke from the exhaust. I restart it and get the same results. Oil level is fine. I removed the OHV cover and a lot of oil mixed with gasoline came out. There are no broken parts visible and the rocker arms, valve springs, etc. appear intact. Any ideas will be appreciated. Thanks