I had a similar problem just when i was getting my 6v 216 running.
The original starter would easily spin the engine w/the head off. I would spin the engine quite a bit in this manner inorder to test oil pressure, circulate oil and with the cylinders full of oil, to try and get the rings unstuck (which did).
Once I put the engine back together the starter worked OK at 1st then started failing to the point it wouldn't spin the engine at all.
It would spin fine in my bench vice. I ended up replaceing it and starting is no longer a problem.
I did take the starter apart and cleaned it up, shined up the commutator and brushes and read continuity of the fields and armature and it didn't fix it. One needs a "growler" to properly test an armature and thats what i figure failed on mine. A good rebuild shop has the proper tools.
Anyways, if your battery and main connections are good and serviceable, you're looking at either rebuilding or replaceing the starter.
I've run 6v electrics for decades on an old tractor I used in all 4 seasons and so far 6v works in my 49 3100 fine too.
Good luck
Dave
