You could also loosen the motor screws, then slide a piece of thin paper between the pinion and gear. Then tighten the motor screws and remove the paper by spinning the pinion gear. This will help in giving you a better mesh between the two gears. If the paper ends up giving too much space and the gears are not meshing fully you may want to look at the wall of the gearbox frame at the motor mount and see if its bent at all which would be "leaning" the motor and pinion into that gear.