OK, I'll accept what you are saying, just because I respect you. But the fact remains that the armature plate must be grounded to the engine. All that stuff about the screw and insulators makes no sense at all. If you could do such a thing, the system would work if you just clamped the plate to the engine with a pair of Vise Grips. Consider the science behind it. The coil produces an electrical voltage. That does nothing unless that voltage can flow in a circuit from the source to the load, and back to the source. The circuit is coil to spark plug wire, to spark plug, jumps across spark plug gap (the load), goes through the spark plug threads to ground (the engine block), through the engine block back to the armature plate, and finally from the plate to where it started--the coil. So, as you can see, the connection between engine block and armature plate is part of the big picture.
The screw and plastic liner are there only to mount the plate to the engine and provide friction to the spark advance/throttle. Has nothing whatsoever to do with the spark. Why it sparks with the screw loose makes no sense---unless that permits some wobble to the plate, which in turn affects the breaker point gap and coil-to-magnets gap.
If you are sure the ground strap is good, I suggest you pull the flywheel and look to see what really is the problem. (Points gap ?????)
Electricity isn't voo-doo, and it follows the rules, which by the way, were established with the creation of the Universe.
