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The black spot is due to the indexing of the plug in the head. The near-side wall of the chamber ceiling is directly behind the darker discoloration, which is opposite of maximum flame travel and indicates a rich mixture in the chamber.
Marking a plug for maximum flame travel will index its electrode nearest to the wall.
Indexing in this fashion is considered by many to be an optimizing effort in building flame kernel in the direction of maximum flame travel without the electrodes strap interfering the kernel growth. Lends to being most efficient.
I was going to say (was - meaning that Dave hit it) that a "proper" gap is probably a undefined gap as related to the service manual since you have changed the components of the secondary ignition. Those coils should dictate a baseline gap measurement, ...and then experiment from there on. Dave seems to have found what works best with his and maybe that should be a baseline for these coils, ...with resistor wire, ...with resistor plugs, ...etc..
Marking a plug for maximum flame travel will index its electrode nearest to the wall.
Indexing in this fashion is considered by many to be an optimizing effort in building flame kernel in the direction of maximum flame travel without the electrodes strap interfering the kernel growth. Lends to being most efficient.
I was going to say (was - meaning that Dave hit it) that a "proper" gap is probably a undefined gap as related to the service manual since you have changed the components of the secondary ignition. Those coils should dictate a baseline gap measurement, ...and then experiment from there on. Dave seems to have found what works best with his and maybe that should be a baseline for these coils, ...with resistor wire, ...with resistor plugs, ...etc..
