Carbon build up is from the castor oil in your fuel. Fuels that are 100% synthetic like Nitrotane and Prime do not carbon up. Depending on your fuel's oil percentage I would tear down and clean the carbon build up off of the top of the piston frequently as well as clean & polish the "button" or compression chamber. Doing this will ensure a clean burn and keep your engine free of pre-detonation.
DO NOT scrape the carbon off, you will have to chemically remove it. I suggest a good carb cleaner or brake cleaner that is acid free.
To polish the button I use Mothers Mag and Aluminum polish (or brasso or the like). Best method is to install an old glow plug in the button, chuck it in a drill motor and spin it up at a medium speed and polish away.