brushes may simply be worn. the starter since it is defective based on your conclusion… you may as well take it apart and replace the brushes and clean the contact surfaces….visit some utubes on how to rebuild it ( assuming nothing else is wrong with it) ..
hopefully when you applied B+ the starter main IN post ……. the starter was properly grounded to the engine frame and the battery B- was hooked up to the the engine ground lug to complete the cirruit.
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