On this bike the cam pushes a little fiber tab mounted on the same arm upon which the moveable contact of the point set is affixed. Thus if the point gap is set too small the tab (which I've called a 'cam follower because that's what it is) will stay far away from the rotating cam, only making contact when the piston has moved too far toward top dead center. The points will open for a short period (called the 'dwell angle') and then close once again as the lump rotates past.
If the point gap is set too wide, the points won't ever close, which stops the spark altogether.
Since my only feeler gauge has blades that don't fit comfortably through the flywheel holes I shall find a piece of steel clock pivot wire about the correct diameter and use that.
I shall adjust and report. The timing light is a remarkable piece of equipment.