The Model number is 944-525800, which I believe is made by Husqvarna. I have tightened the spring tension the to point where the belt will just freewheel when the engagement lever is released. Should I install a stronger spring? There is a possibility that the belt continues to slip even when the lever is fully squeezed due to insufficent tension on the idler pulley. Is this a common problem with this model?

well, lessee how this looks: "30.8. Slip the crankshaft pulley down far enough to get the belt off of the pulley, and remove the belt from the crankshaft. See Figure 30.8. NOTE: Belt keepers that are part of the tractor frame prevent the belt from being removed without lowering the pulley. NOTE: The pulley may be removed from the crankshaft at the discretion of the technician." That's right out of the 1000-1500 series serv manual.... and I don't really know if there is enuff room to lift thet engine straight up with the ground drive pulley in place to be able to wangle the ground drive belt off past those fixed keepers - which I fully agree are something of a PITA. just saying... MHO.

I don't recall that you mentioned this detail. It will cost you nothing to check it. You did ask for help. The only power transfer is between metal drive disc on the crankshaft and the rubber tire on the driven wheel then between the liner on the inside of the driven wheel and the drum that it rides on. There is no pressure plate. The fiber disc is, as I said previously, simply a spacer NO power is transmitted via this part. There is a Mirad of things that could cause your problem from sheared thru bolt in one of the hubs to worn out shifter yoke or grease on the drum where the liner runs. Things do wear out and slip adjustment. I am afraid you will have to do more checking to pin point it. Might start by standing the mower on its' rear standards with shifter in neutral, grasp both wheels alternately push and pull on them then one at a time Wheels should feel connected to each other when both are turned and driven wheel should spin. Holding one wheel while turning the other should make held wheel want to turn backwards and driven wheel spin. While there check the position of the stop for the shifter yoke, check for worn dips in the shifter yoke where the roller on the center chain case runs back and forth to change speeds also to raise and lower the center chain case with the rubber tired driven wheel on it. This should be basically flat, no deep hollows in it, check for broken or weak pressure spring on the shifter yoke for starts. Walt Conner

Reverse the Photo-Eye Units and see if that helps. That usually fixes the problem when we encounter it. Sunlight is a problem for photo-eye emitters & receivers