Theres a spiral on the shaft of the starter where the drive rides.. It allows the drive to mesh with the flywheel teeth sorta like a syncro. In your case the drive is spinning before it meshes with the flywheel teeth..So in other words it is moving too slow..Could be the spiral is gummed up or the drive is worn ??...I would take it apart and clean everything and replace the drive.. Check both bushings.A dragging armature might act this way.. Make sure the linkage from the foot pedal is not too worn or binding...
