Most of the hydrostat setups have a way to adjust the tension on the speed selector. May be located on the transmission, could be on the speed lever that you move to go, or on some units is located within the linkage between the speed selector, and the tranny.
Will look like a nut on the end of a threaded stud with a spring and some friction washers underneath, that moves within a slot.
I did see two of those...one was the brake adjuster, the other is likely what you're talking about. I'll take another look and trace the linkage to see what it moves with.
Thanks again....
Jack