You connected the high voltage lead and opened a valve to increase air pressure in the box.

The plug would keep arcing right up to full line pressure, where a dirty plug would not.

No longer available because "A" few people know what a spark plug is, and "B" Lawyers.

Lynn E. Hanover

Or it could be:

A) few people know what a spark plug is

B) plugs now last 6 to 8 times longer with unleaded gas

C) The sand blasting pitted the ceramic insulator so the plug fouled twice as fast the next time

D) Plug manufacturers didn’t want people restoring old plugs instead of buying new

And/or E) lawyers J

Al