sounds like your dieseling- are you using alcohol gas? they did not have that when your tractor was built- I never use ethanol gas in my equipment- I ALWAYS run my equipment at just above idle for at least 5 full minutes before I shut it down -lets the machine cool down -newer engines have that fuel shut off soloniod on the carb to prevent deaseling- for carbon build-up- you might want to try marvel mystry oil in the fuel-