Murray, does it use a pair of variable diameter V-pulleys to get the speeds? If so, the gear selector linkage must control the diameter of the master pulley. In other words, if it works as I'm visualising, it sounds as if the problem is either a linkage adjustment, a jammed expanding pulley mechanism, or a worn belt that limits the possible range of pulley size.
ken, does your mower have a variable ratio V belt drive like Murray's?
