Lawn tractor backfire

Try letting it idle for a minute before you shut it off. When you shut it off the only thing you take away from the running process is the spark. When it is shut off the engine has a coast down time before it quit turning. It still intakes the air/ fuel mixture, compresses it, it doesn’t burn it because the spark is turned off, then it exhausts it. This unburned fuel mixture is pushed into the muffler, if the muffler is hot enough it ignites it and thats where you get your “backfire”. By idling it down before shutting it off you accomplish two things. First the engine coast time is less because its not running as fast, second the throttle plate is closed so we can’t get in as much fuel. Hope this helps, Joe