I put new spark plugs in my '51 (216) about 2 years ago. I'm ashamed to say that it has only been driven about 20 miles in that 2 year period. (I'll do better!) I have been working on it lately, and I feel that the motor is missing somewhat. I pulled all the plugs, and most are really covered with black carbon, to the point that I don't think they all are firing. I have ordered new plugs and will replace them, but, was just wondering what causes that black buildup, especially considering it has so little running time on it since they were put in new.
1951 3100 Chevy Deluxe Pick-up (original with 73,000 miles on a "straight Six" 216)