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Is this pulley tilted, or the one back to the right, not shown/ If either are not straight and square, belt will walk off. A bad bearing on either pulley would cause pulley to tilt.
Run a straight edge over the top f the pulleys, looks like the right pulley may not be level.
With belt riding up on the right pulley, and in the center on the left pulley, seems to be something is not straight! Belt is walking up the pulley.
Shaft for pulley or the arm to it, is not bent? For belt to ride up, pulley needs to go more left at the top. Can the bracket on the mount, right side, be shimmed to change the angle?
I was thinking more like tilting the assembly, and you could do that by removing one or two of the shims.
OK< I will opt out. Maybe someone else can help. Please do not respond to me, or it will lock the question. Sorry.