I bought a used HR215 (from original owner who bought it on 5/20/1994). Got it home and changed the oil, cleaned the Carburetor (just sprayed it with carburetor cleaner, didn't take it apart) and put the carburetor back on. The issue is the engine will only smooth out when it is in full choke (meaning throttle lever all the way forward), if I pull the throttle lever back a little the engine surges. By the way, the GXV140 engine doesn't smoke (no blue or black or white smoke out of the exhaust). The spark plug was dark (probably carbon build up) but is now starting to lighten up. Any ideas on whats the cause is with the surging issue?