Carbon dosn't happen as much now as it did in the past, tearing my engines down after 3 years of running revealed on slight build up. engines run as a mower that cuts grass tend to build up, some pretty bad, usually a ring issue as factory tolerances suck. One trick from back in the day is to get the engine operating temperature and squirt water into the carb GENTLY, this will turn to steam and complety clean the combustion area. Used to have a 1973 Thunderbirs that had to have a coke bottle of water every 4 months, it would sometimes knock so bad (from the carbon hitting the head) that I was afraid to drive it. A bottle through the carb as it was good as new. Had a Ford V-6 that blew on one side and when the head came off, it was absoulty spotless. Just do it in moderation