If the spring rubs the shaft, it will wear a groove in it. Clean the crud out of the hole with carburetor cleaner and shine a light in there. If you can see the shaft turning, the plastic piece is missing. Sometimes it's very hard to see unless you do a complete teardown, including removing the drive gear and taking the shaft out of the housing.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!