The carbon tracks are often hard to spot on the plug wires. Mine were inside the boots on the plug ends. However, the porcelain on the plugs was also scored and tracked, so that was a dead giveaway.
Anyways, the boots being black, and the carbon tracks being black, they are hard to see.
To repair it, you must replace everything. If you have tracked wires, they will burn up a new set of plugs. And if you have tracked plugs, they will burn up a new set of wires.
Buy all new plugs and wires, and change them all at once.