hey guys had to get the old swz scag walkbehind out to finish some yards. oil looked good havent used in a while. cranked right up mowed two lawns no problem. on the thrid was mowing along and then all of a sudden revved up super fast. like i thought i was about to blow the engine was going so fast. tried to idle down nothing. cut it off. tried to crank up again and it was doing same thing like running wide open like fast real fast. choked it down with the choke and loaded on trailer. any help would be appreciated.