So if I understand it correctly, you had a 6 volt starter running on 12 volts and you had to disconnect the battery to get it stopped? After letting off the stomp pedal I assume it was running full out with no load. I am surprised that It didn't completely self destruct. Maybe it is drawing too much current and that is why the contacts welded. The six volt starter should be spinning quite fast with 12 volts and the drive engaged. If it is not turning fast, the current on 12 volts would be excessive.
