Assuming the heads, block, crank,ect are true . The only thing that will make the belt run off is the Idler pulley ! The belt will walk to the tightest side of the pulley . I took the spacer behind the pulley and used a fly cutter to take .030 off . This put a angle on one end of spacer and flat on the other end that goes up againist the inner race of bearing , this will let you change the angle of the idler pulley too make belt true . I'm a machinist , and it took a couple of try's too get it right You will also need to do the same thing to the washer or spacer on the backside of pulley bracket with the same angle you have on the spacer that you just cut ! Unfortantly blower manufactors are not belt people .
Like the other guy said bearings must be good or you will never get it right ! Also , turning the belt around , or replacing does not work ,belt will still walk towards the tight end of pulley ! Hope this helps . . . . . YOU CAN EMAIL ME IF YOU NEED MORE INFO !