The sparking is comming from the commutator. You should buy a new armature (the complete rotating part of the motor) you must fit new brushes of the correct type for that paticular motor. Bed the brushes in by wrapping some fine glass paper or fine emery tape around the commutator with the brushes in place rotate the armature back and forth to get the brushes contoured perfectly to the comm. If you have the old Armature tou could try first to undercut the insulation between each comm segment using a stanley knife. Dont take the level down very much and clean the comm up afterwards. You could also skim the old comm on a lathe. Most likely you have an open circuit segment on the old armature. If there are any suppressor's on the motor it would be a good idea to replace these. I hope this helps
R Sutcliffe
June 2006
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