I have an MTD lawn tractor seems to act like it is sliping if forward gear I have repaced the belt. Does not do this in reverse. Could it be the transmission is that rebuildable?

It should be adjustable... Does it have a varible drive perhaps?

Yes it does it has a pedal that varies speed.
Thanks

Seen many people have problems with these vari drives.. and may be the cause of the slipping... all belts replaced right?