Son bought a new house. Previous owner left him a 10 year old Craftman Professional series 357cc snow blower. Snow blower starts up and seems to run fine as long as you have choke on. When taken off choke and moved to run the engine revs to the point it sounds like a jet engine. Engine model # is 483-SUA. Snow blower model # is 247.888301. Oil is dirty and somewhat over filled. Do not want to blow the engine up. Any ideas whats going on.