I have a Craftsman 15.5 HP lawn tractor Model 917.256810. The belt drive is loose. How can it be adjusted to tighten it?

Hi,
There is not an adjustment, per se. There is a spring that tensions the belt by pulling the clutching idler to the right when the pedal is released; I would check the health of it. The manual is here:
Owner's Manual - CRAFTSMAN Lawn, Tractor 917256810 - ManageMyLife.com
And page 22 is the page needed. The sping hooks through the right frame.
Peace,
Clarke