Originally Posted by
quietflyer
Seeing the bottom photo, and the brush on the 'left' tells me couple of things to consider.
1. The brush is well worn.
2. That that brush is so worn, that the spring tension holding the brush to the commutator is not strong enough to 'hold' the brush onto the surface firmly.
This would indicate that current flowing from the brush to the commutator is not enough/ or intermittent, thus the 'intermittent' working of the starter when engaged.
Suggestion, obtain a new set of brushes and springs, install. Most likely that will solve your problem.
Mike