I can't remember the reason for using the "test spark plug gapped at .010".  Lynn, would you cover that again?

At high RPM the amount of time a Kettering coil has to saturate, or develop a full flux field before the primary field supply is opened to fire the coil on field collapse. So, the output voltage of the coil drops with RPM. So when you need great coil performance, you are getting into the poorest range of the coil's performance. In order to eliminate low secondary output as the cause of a high speed miss, you shorten the gap so as to reduce the voltage required to fire the plug.

 

So, if you make no other changes, and performance is restored on shortening the gap, the problem may be ignition performance related.

 

High energy systems do not build a flux field in the coil, and then wait for a field collapse to generate the high voltage to fire the plug as in the Kettering system. The high energy system applies 350 volts to the coil primary from a fully charged capacitor. The coil fires as the flux field expands through the secondary on the way to saturation. The high primary voltage overcoming the inductive reactance of the primary that acts like a resistor in the 12 volt system. When the primary circuit is opened, you get another hit on field collapse (from a fully involved flux field) that is like a Kettering system at idle. Some systems let the coil primary and capacitor ring like a simple tank circuit so as to keep pulsing the gap once an ionized path is established. 

 

There is enough excess energy at the plug gap that an arc may develop to a ring or wrist watch held close to a plug boot. This will not be the warning sting as from the lawn mower.

This will be the micro nap on the way to the ground, where you will rest a while and try to remember your name.

 

If you have Stents or a Pacemaker, it could be fatal. Modern automotive systems are so labeled.

 

Lynn E. Hanover