Tom R,
I think the only item "switched" with the key, that would draw battery current through the ammeter would be the ignition. If the points were closed, and the key left on, the coil would continue to pass current to ground and heat up.
I thought the wire from battery to ammeter was pretty large, gauge wise. For this to smoke, it would seem that anything smaller would have fried.
Have you tried to start the rig after this problem, and did it start? Are you sure it was only a "key left on" problem?
Normally this scenario results in cooked points and/or a burned out coil.
Stuart
