Carbon comes from gas that isn't burned completely. Usually from putting too much gas in through the carb.
You'll always have SOME on an old tractor.
Are you leaving it choked when starting it? Open the choke up immediately when it starts.
Also might want to try leaning the carb a bit.
If you do that - be sure to test it under load to be sure you didn't make it too lean. And also check your plugs occassionally to be sure you're not running too lean.