You need to take the end cover off and expose the Commutator and brush gear.
Look for build up of carbon and blow out if necessary, and commutator appearance, it should be smooth with no pits are sign of severe arcing.
Look for overheated/burnt armature windings.
You could try running it on a automotive battery, it should run up to speed easily with no growling or arcing etc.
Al.
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CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.