Had that problem with our 1660. First things first, leave the machine outside away from buildings etc. Under the hydro pump there is a small lever that your hydro stick links to This little lever is attached to the hydro on a small splined shaft. MAKE A MARK ON THAT SPlINE WITH A SMAll SCRIBE. Ours was worn bad, made a groove in the lever and we lost ground speed. I just rebuilt it with a little welding and made it square with a bench grinder. The trick is to re - install it. The small splined shaft on the hydro pump is spring loaded. This is why you need to make that mark. This happened a few years ago so I hope my memory is correct!! The reason I suggested parking outside, is because is you do not get set properly the first time and you start the combine, it may move either forward or reverse.