Alan.just to put your mind at rest about the Rf gremlins you refer to.
This is RF generated from within, not from the outside.
In a Capacitor Discharge unit, there is a very high voltage spike emitted and it
Must be screened metallically from the plug right back to the ignition unit output.
If you have the basic magneto system the initial Voltage spike is very much lower, just like the the old Kettering ignition system
and screen leads are not required, just a resistor plug. That insulation that you referred to is just that, an insulator to prevent the spark fro the inner cable being able to jump to any outside item. It has nothing at all to do with containing RF emissions.
Hope this helps.
Cheers
From your friendly gremlin killer