Sparking at brush-commutator can be a result of a LOT of things. Some are electrical, some mechanical, and others environmental.
First question - what happened recently that made the sparking appear? Or why did you think you needed to adjust the neutral setting?
The following list is by no means exhaustive.
Electrical: excessive load (current or voltage), poor contact resistance (film), bad electrical connection (bolted joint), turn fault (interpole or main pole winding), ground fault (interpole, main pole, or armature winding), shorted bar(s) - at commutator surface or at riser, brush contact position incorrect with respect to machine electrical neutral, open circuit in one of the machine windings, etc.
Mechanical: brush arm circumferential spacing, brush arm skew (relative to commutator bar), high runout on commutator (bar-to-bar), weak brush spring(s), brush fit (to holder) incorrect/loose, brush fit (to commutator) incorrect, surface speed too high, commutator bar relative movement (loose commutator and/or excessive speed causes bars to "bow", possibly settling back to proper place when at rest), induced vibration (from process or alignment), incorrect circumferential spacing/alignment of main and/or interpoles, deformation of running surface (grooving, threading, high/low spots) etc.
Environmental/Other: Contaminant in film, presence of oil, abrasives, chemical reaction between atmosphere and brush compound resulting in increased brush wear/carbon dust formation, presence of silicone caulk (e.g. RTV) in air stream, incorrect ventilation (commutator surface too cold or hot), accumulation of conductive debris (carbon dust, etc.) in undesirable places - such as behind the risers which may result in shorted connection from bar to bar on the commutator, etc.
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