If you don't know the history, there's a good chance the previous owner might not have kept up with regular oil changes. I know the guy I bought mine from didn't.
He said it would lose power for 15-20 seconds and smoke out the exhaust. My guess was a sticky ring. I added Seafoam to the oil through the crank and let it run for 5 minutes, revving it off and on. Right after that, I changed the oil. I have yet to have any problems like the last owner had, and it runs quite smoothly.