Here is an old post by Hot Rod Jerry. It goes to show that everybody is saying the same thing that he said. Independently.
"OK- - - -so the starter armature spins, but the flywheel doesn't, right? You're going to need to do a little more troubleshooting, as there are two very different situations that can cause the problem. The starter drive has a one-way clutch built into it that can fail, allowing the starter shaft to spin with the drive gear engaged into the flywheel ring gear, but not turn the engine. The other possibility is worn linkage pivot points that allow the starter armature to begin turning before the gear gets to the flywheel. Does the starter grind occasionally? Or, does the engine start normally on occasion?
Take the sheet metal cover off the bottom of the flywheel, shine a bright light up toward the starter, and have someone step on the start pedal. You should be able to see if the gears are meshing properly. If so, and the engine doesn't turn, replace the starter drive.
Jerry"
