[greglepore]

Well, it could be any of the above. It could be carbon, but on a highly mod'd 930 with serious track usage, I'd suspect anything from a lean condition in that cyl that burned the valve or a mechanical overrev on a downshift that just barely nicked the valve due to float. I'm discounting the carbon b/c the way your car is driven its not particularly likely.No real way to tell until you determine why the valve isn't sealing, then work backwards from there. Good argument for a cht gauge, if you don't have one already.