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Tom --
Others have described cleaning inside the "air valve" part of the motor with spray carb cleaner while flicking the armature around. I think this "degunks" it, but can't overcome a worn out motor.
This motor is different from a conventional motor (that can spin 'round and 'round endlessly). The armature in this motor can only rotate back and forth about 90 degrees, open and closing the small air valve.
I've pulled several apart (it's a b*tch to do) and cleaned and reassembled them. However, on mine, it did little good. The armature has a three-sectored commutator and three little brushes. The ones I explored had the copper commutators worn through (eroded or burned, more likely) at the exact spot where the armature wants to be during normal operation. Thus, a dead spot. Whack it with a stick, and it'll work until it again hits the dead spot.
If you pull one apart, be observant for a small "funnel-shaped" washer on the far end of the shaft. It must go back. It spreads the brushes when you insert the shaft back into the bearing.
I suspect that premature failure might occur if the engine has a lot of blow-by (perhaps from many miles with a clogged ventilation system, as was the '86 I bought). The CIS motors I disassembled were dripping with oil inside, around the brushes. I believe this caused burning and carbonization of the copper commutators.
On my '86 (with about 200k miles) I installed a new one -- big, big difference. I think it was around $125-150, more or less.
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