the motor out side of the frame you can rotate the armature freely no bind. I put dc to the motor and it moves vary slowly at full throttle. I changed the springs and brushes with atlas set. and motor still the same.
Are you able to measure the current draw of the motor outside of the frame while it is running at full throttle?
Can you sharpen a toothpick, like I mentioned in my earlier post, and see if you can clean out the gaps in the commutator? If those gaps are packed full of oily carbon dust, it can be conducting electricity, shunting the commutator segments, and making the motor run poorly.
If that doesn't do it, I don't need another project, but I can't resist a good puzzle.
Would you be willing to mail me the motor so I can try to figure out the problem? If not fixable, I can put its flywheels on one of my spare motors, and ship a working motor back yo you. No charge - in the name of science!
