I have an '07 troy built pony. It keeps burning the belts off. Seems to slip only when the pto is not engaged. There is no tension on the belt. The smoke is coming from the crank pulley.

If smoke is coming from the crank pulley then the belt is to tight when PTO is off. It sounds partially engaged by too much tension or you may have it on wrong.

A new belt is not stretched and an adjustment on the tension pulley would be my attack. Even though the springs on the tensioner move some its position starts in the wrong position because of a new belt being a tad smaller. Like a tweak for an alternator when a new belt is put on.