[Al Skierkiewicz]

Andy,
Arcing in brush type motors is normal. It is occurs when the brush just leaves a commutator segment, opening a winding in the motor. The collapsing magnetic field generates a high voltage which jumps across the gap from the commutator to the brush. At high RPM and high loads, the arcing is at it's worst. As the motor wears in, the arcing also grows worse as the arc is able to jump across (through?) all the debris built up on the commutator and brush.