Spica Starter Motor Run-on/CSS Feedback Loop
In the Spica system the Cold Start Solenoid (CSS) is energized from the key switch connection on the starter solenoid. It is a common problem that the high current draw CSS sends an electrical feedback to the starter solenoid and causes the starter solenoid to "stick" on and send continuous electrical current to the starter motor.
When this happens, the starter stays in engagement even if you turn the key switch OUT of the start position, or even to "OFF" altogether. As you can imagine, this could be a big problem perhaps resulting in a grossly overheated starter or damage to the starter pinon or pinion ring on the flywheel.
Fortunately, there is a simple solution. Simply re-locate the feed end of the CSS wire from the starter solenoid key switch tang to the starter solenoid main output/starter motor input terminal. This prevents the CSS feedback from entering the command section of the starter solenoid and prevents possible starter run-on.
Below is a picture of the modification (from a 74 Spider).