Can you try a different fuel? The only time I have experienced build up of carbon is when there was castor or some other carbon based lubricant in the fuel. Even in small quantities the carbon based lube will leave carbon deposits on the exhaust valve/surfaces even the piston at times.
I have run Wildcat YS 20/20 pure synthetic in all my YS engines with good results. No sign of carbon deposits and engines have run flawlessly for many years.
Dennis