Hey, on my 3204 the outer bearing when out on the pto shaft, maybe yours are going and cause the pto shaft and pulley move making the belt come off. On the motor are the belt guide in place and on the bottom of the pulley on the pto shaft there should be two bolts that keep belt in place. this may help to,
http://www.partstree.com/parts/?lc=c...&dn=0055200002