First I will say that I have resolved this problem so it is to what other would think the problem was. I had overlook an obvious root cause of the problem. It me four hours overall to fix it but should take only 1-2 hours normally.
Brian I have already gave you hint what cause this in a unrelated PM so let's see what others think first.
The troublemaker: 2001 Huskee 13AM660F131.
The problem: Customer calls about mower not moving. Upon putting the variable speed to transmission back on things seems fine. While testing the mower belt come back off three times. Everytime it was when I was engaging reverse; never in forward drive.
Know good parts: The belts and idler were replaced two years ago. Belts and idler are aftermarket parts. Still in very good shape other than normal wear.
Now what would you follow techs would think the problem was?
AVB