On an air cooled motor running at anything but WOT throttle only, you can expect the outer ring to be black always. That is, if the cooling fins are doing the proper job of cooling the engine. Then the temps at the base ring will not support combustion.
Have you gotten past the initial oil consumption of ring break in? Those deposits could brown plugs some, I think.
I would look into the insulator. The base of it could be carboned, too. Perhaps up to a third for multi throttle positions.
Slightly on the rich side helps cool the engine during break in while the friction is a bit higher.
Cheers,