[OrrieG]

Sounds like the bendix (part on nose of starter that engages the gear to the flywheel) is bad of solenoid is bad. Solenoid is energized from the purple wire, when the circuit closes it pushes the bendix out to engage the flywheel and starts the starter spinning. When the bendix hits the right rpm that tells it the engine is running it disengages and closes the solenoid circuit shuting off the started. Might be as simple as the starter being shimmed to close to the flywheel so the bendix binds and cannot return.