Scott,
Tell me where the trace is. Does it lead from the cap down the side over the porcelin to the steel hex body of the plug? Or does it end short of either cap or base? What type of grease have you coated it with last? Do you also put grease in the rubber end of the wire? Are there sooty places on either the outside of the rubber, or from inside to outside on the end of the rubber boot?
What plug are you using?
The things I want to explore are cracks in the porcelin, and why they form.
Faulty plugs that may not conduct to the spark tip, making the trip outside the plug more desirable.
The boot.
Facts (and please excuse me if you know these things already, I don't mean any disrespect for your knowledge)
It's tough to break down air at the gap to the point where it can form an arc.
The larger the gap, the harder it is to arc.
The energy will mindlessly take the easiest path.
From the very first trip down the wrong side of the plug, that pathway is more desirable than jumping the gap.
Each additional trip makes it more desirable as any material that can be heated to a carbonized state improves that path's conductivity.
Give me some more background, and I think we'll either conclude that the porcelin is getting cracked, or the grease has already become compromised and needs to be completely removed, or the boot is damaged and providing the pathway, leaving a misleading mark down the side of the plug.
In the electrical repair world I have observed, high voltage circuits are designed specifically to minimize potential alternative paths. Dust is often the initial culprit. Even household voltages can lead to carbon tracks. Outdoor circuits where contaminents settle on a surface, and moisture is present will form tracks. Once the intended insulator has been burnt, it is often nessessary to cut it completely out to eliminate any remaining carbon.
Gar
