This video shows the carbon tracking on a bad spark plug found in a Subaru Forester that came in for a poor running condition / engine miss. Carbon tracking occurs when the ignition spark travels around the outside of the plug instead of jumping the gap in order to make a spark. This condition was fixed by replacing both the plug and the spark plug wire since both were ruined when the ceramic and rubber boot were burned. The condition mostly occurs due to extended high mileage neglect of tune-up components.
Video provided by Forest Automotive: Complete Auto Service & Repair in Lynchburg, VA and Forest, VA