Small Engines (Lawn Mowers, etc.): Lawn Tractor Transaxle, input shaft, transmission belt

Question
I have a 1991 Craftsman II 12.5hp 6 Speed 38in. model 917.255570. Transaxle busted, bought a used one had to modify mounting holes but basically the same trans, I don't know what it came off of. My problem is when the mower is running you cannot shift gears, turn the engine off and as soon as it stops you can shift. Belt seems to be loose enough. ? What effects the shifting?

Answer
The problem is the belt.  When you depress the clutch the transmission belt is supposed to stop turning the input shaft on the transmission.

I have seen this problem before...the belt may not be turning the pulley but it is putting just enough pressure on the pulley to cause the tranmission input shaft to prevent the transmission from shifting.

Did you use the orginal pulley off the orginal trans?  Can you move, temporarily, the trans forward to see if it helps the shifting problem?  What belt are you using?  I've had problems using aftermarket belts.

Let me know.
Eric