There are two allen head screws on the disk attached to the motor. Loosen both (don't take them out) then the disk will slide on the shaft. Put the mower in first gear, and drop the motor disk until it touches the drive disk. then tighten the screws.
Depressing the clutch slightly and dropping the motor disk will give more pulling power but you run the risk of rubbing your motor disk, which is a bad thing.
Kurt, May 2007