It could be a couple of things;
1) the carburator float may be gummed up letting fuel flood the carburator and engine. I would remove all fuel lines, replace fuel filter, inspect and clean the diaphragm in the fuel pump and remove and dissamble the carburator. Clean the jets and submerge all metal parts in a carburator cleaner. Re-assemble the carburator and fuel system and get a tankful of fresh gas. 2)if the engine is not producing a spark it will not fire and by crancking the engine you will cause it to spew the unburned fuel out of the exaust port. Test for a spark, attach the spark plug cable to a loose spark plug. Lay the terminal end (bottom) against the cylinder head. Do not hold with your hand or metal pliers as you may get a shock. Then pull the cord, you should be able to hear a "tick" and see a bluish spark across the spark plug gap. If you cannot get a spark replace the electronic module. ($$) 3) The third component necessary to fire an engine is compression. With the spark plug removed place your finger in the hole and pull the cord, you should feel pressure and vaccum as the engine cranks. If no compression it could be stuck valves in a 4 stroke or worn rings. Big ($$$) Good luck.
