Small Engines (Lawn Mowers, etc.): Sears L8 Tractor OLD, pulley system, speed ratio

Question
I have an old sears tractor that runs very well.  Now when I go down a grade the tractors slips in a jerky manner.  I changed the belt since that was easy but no dice.  I was wondering if the differential could allow the wheels to "free" wheel.  The differential is $70 pls shipping and I didn't just want to swap parts and see if it works.   It happens in every gear and the clutch brake does not stop it either so not safe.

Thanks for any advice.

Tom

Answer
Hey Tom
Lemme see - on your tractor you have a pure differential, with speed ratio's determined by a variable width pulley system.

Definately your differential could cause this, but just to make sure it isn't something simple make sure you check the SPRINGS that set up your speed pulleys underneath.  They also definately cold cause this.