Lawnmower was running fine, stopped in front yard to go in and eat, and when I came back to start it it cranked over 3 times and stopped. Done the same thing twice more then it back fired smoke came through the carborator. Does anyone no what's wrong and what I need to do. Its an 18 1/2 HP

Hi,
First guess is that you shut it down with the engine revved up and you sheared a flywheel key.
Peace,
Clarke