The reason your tractor will start and shutt off is from the fuel selonid on the carburator.
I have a cub 2185 that did the same thing. My repair for the problem was to take the selonid out and grind off the end of it. That will let in the fuel and problem solved. The only reason that the selonid is there is to stop back fireing or run on when the engine is shut off.
My 2185 runs just fine with out the seloind.
