Curtis,
The purpose of the missing set screw is to prevent the gear walk problem that you are having. The screw should be a #8-32 x 3/8" socket head cup point.
If you haven't recently oiled the spindle pulley bears, do that first (remove the oil plug set screw that you will find at the bottom of one of the two inner belt grooves and place several squirts of oil down the hole, replace the plug and confirm that the pulley spins freely). To properly locate the bull gear, trim a sheet of printer paper to a width that will fit inside the headstock. Cut the lower corners off and cut a slot the width of the spindle diameter from the bottom up about 3". With the lock pin in the OUT position, slide the sheet into the gap between the bull gear and the spindle pulley. Press the bull gear lightly against the sheet and pulley and tighten the (currently missing) set screw. Remove the sheet and start the motor (do NOT engage the back gears). The pulley should turn and the spindle should not. Confirm that the pulley is spinning without touching the bull gear. If it is rubbing, stop the motor and repeat the procedure with two sheets of paper shim. When finished, lock the bull gear to the pulley with the pin.
Robert D.
