I recently replaced my Agility 50 engine with a 139qmb ( I blew it up - a long and boring story ), and the only difference in the stator is the a50 uses a 3-wire stator ( white, yellow, blue/yellow ) and the qmb adds a forth red/black wire that goes to the ac/spark on an ac only system. Since the a50 runs the spark off of the battery it doesn't use the red/black wire, but it works if you just connect the other three.