So I get home with the new W9 DC plug and begin to remove spark plug #4 (the wet one) and lo and behold it isn't wet! What happened? The other day I had made a small timing adjustment and advanced it a tad. Obviously it was just enough to allow cylinder #4 to achieve a proper burn. I installed the new W9 DC anyway as an experiment and will take it out for examination in a couple of days. I'll probably end up putting the W7 DC back in but it's good to know that the engine is behaving normally.
1967 Jaguar E-type Series I, 2+2