Ringed engines should NEVER EVER be used as Diesel conversions in the air. IMVHAIO. Ringless pistons are the proper way to go as is the proper fuel. Synthetic oils have NO place in model diesel fuels. Set the engine right and you shouldn’t have to do much to it for a long time. Carbon buildup in a short period of time causing the engine to not run properly leads me to believe either improper engine settings or incorrect fuels being used. IMO.