It's great that you found it. We used to have the same problem with 2 cycle snowmobile engines. All of a sudden, one cylinder would start running sour. We'd pull the plug, and when we looked at it, it seemed perfect. However, if we replaced it with a good plug, and it would run like gangbusters. A friend of mine had a spark plug cleaner and tried to resurrect several of these"dead" plugs; to my recollection, they just wouldn't come back.
