When was the lasted time you changed or checked the drive belt? A worn belt and/or worn idler tensioner pulley are the most common causes for this problem.
#3
Scrubcadet10
what's your model # it will be under the seat on a tag, starts with 13_______________
Ditto with Rivets
park on the top of the hill and release the brake
IF it stays there the hydro is good & the belts/pulleys are bad
If it races a way like mulga Bill's bicycle then the tranny is gone & needs a new motor - pump unit .
And of course partial wear between these two extremes
#5
kbowley
Does it happen when you first start it or only after using it for a while?
