I have an old Cub Cadet 1620 hydro and when I start moving it goes great. Then it slows down and will hardly move. It mows great, but it will only go a few miles an hour. Any ideas of what the problem is?

Is the transmission fluid level correct?

Will the tractor will go in reverse with good torque? If so, the pump is probably fine and a linkage problem may be the culprit in the forward direction.

Hi,
Link here for the model:

cub_cadet1620&dn=0027600005 (http://www.partstree.com/parts/?lc=cub_cadet&mn=1620&dn=0027600005)
And I would guess that an idler, spring #30, or bushing #12 was not lubed at fitting #31.
Peace,
Clarke