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I have a scag 36" walk behind that keeps throwing the drive belt. Replace old belt and cleaned all pulleys and they all look to be aligned.

If I was to guess as to your problem I would say you installed the wrong style belt. Buy your belt from the dealer. If you have done so, you must have a pulley alignment or bearing problem.

If you have an OEM belt, I would check the bearings on the idlers as reynoldston said.

And also check the belt guides.