Back on the transmission, right side, is an arm that is attached to a shaft on the transmission. The arm is the last piece of "#34;linkage" that the hydro lever moves when operating the lever. The shaft goes inside the transmission and controls the swashplate, which determines piston displacement and ultimately the speed of the tractor. The arm is held on the shaft by a roll pin. It is very common for the roll pin to break inside of the shaft, allowing the shaft to rotate inside the hub of the arm without the hydro lever on the dash being moved. This will cause the unwanted acceleration that you mentioned. I've now seen 3 or 4 tractors that had this problem. It's easy to see the arm loose on the swashplate shaft if you are looking at it closely or holding it while you move the hydro lever back and forth.
Also check for any other undue "slop" in the entire hydro linkage, from the "orange knob" all the way back to the transmission.
(Message edited by dlnw98 on November 26, 2005)