Do your adjustments to the mule drive belt first, then run the mower to see how it performs. If the belt still slips, then I'd replace the tension spring. I forgot to mention, the idler pulley on the mule drive should be keeping the belt tight when the pto is activated. When not activated, the mule drive belt should not be turning. Check to make sure your tension is set correctly for the idler pulley, since you said your mower deck blades still turn when the pto isn't activated.
This would be so much easier to explain in person. Just where is Kittanning located in PA anyway? We should just meet some Saturday, and then I could show you all the in's and out's of these old round fenders.
Edited by johndeereelfman, June 17, 2014 - 09:40 PM.