Reversed universal universal motor sparks and shows arcing even though brushes are completely perpendicular to the commutator
In an ideal motor, the ideal position for the brushes, the commutation plane, is perpendicular to the magnetic field.
In a real motor, with finite inductance, it is rotated a little with respect to the field.
This rotation may be ignored (especially in small motors), or be achieved either mechanically, by moving the brushes, or electrically, by running the rotor current through small auxiliary windings near the brushes to 'tilt' the stator field.
The surface condition of the commutator controls a surprising amount of voltage drop at the brush/copper interface. Deposits may have built up favouring one direction of rotation. If all else fails, try cleaning the commutator surface with very fine grit paper, remembering to clean all the copper dust from the slots before trying to use it again.