When the engine starts (or fires even just once), the flywheel is turning faster than the electric starter can spin the starter armature (bendix gear), and therefore the gear is kicked out downward and there it sits until it is suddenly engaged from a dead stop..... that sudden burst of activity is what sends it upwards.

However... with the engine running, if that bendix is actually being turned by the electric starter (Not Vibration), the starter solenoid is most likely shorted out, in which case, replace the solenoid.

The only other cause would be a shorted ignition switch (not likely).

NOTE: Faulty, shorted solenoids are not a common problem BUT it happens. This type problem is caused by using a very low charged battery OR a poor quality battery. Such a battery causes the inner plunger of the solenoid to pull in weakly (instead of slamming in) which results in a lot of internal sparking which eventually causes the plunger to stick to the inner contact points.

Tapping on the solenoid usually cancels that problem momentarily.

Joe, thanks for your reply. Yes, my bendix does drop down below the flywheel ring gear, but continues to spin; and spin; and spin — rapidly. I did have a problem with the solenoid "rapidly clicking" when the key was turned to start. So, I disconnected all wiring in the starting system, cleaned all the connection points and fittings, and reassembled. I also charged the battery overnight. When I tried starting, I got more clicking a couple of times; then, the starter did engage; and, did so several times. Does this seem to confirm a faulty solenoid? Thanks, again.

You - Does this seem to confirm a faulty solenoid?

Me - Sounds like it to me. Best procedure is if it sticks one time... replace it.