I turn the key from on to start, the motor turns but continues to turn when i bring the key back to on. Help?

It is on a 66 lincoln continental. I have replaced the starter which comes with the solenoid. And replaced the key and tumble, and ignition switch. When you first put the key to the on position it does not start, then you go to the start position, listen to it turn, then go back to on position and it continues to turn. Please give me suggestions