On a certain motorcycle, from 2012-2016 the start motors are notorious for working poorly. We have investigated numerous causes (batteries, grounding, cables, relays), but have found that swapping starters with known good starters usually fixes the issues. So I have disassembled a few starters to see if I can find the internal problem. One interesting thing I have found is that there are no alignment marks on the case so that the orientation of the brushes to the permanent magnets is not consistently aligned.
I do not know enough about DC brushed PM motors to know if this is really an issue or not. Is it? Could this be our culprit?
I have attached a couple of photos of either end of the motor.