Hi!
The carbon build-up you speak about is a symptom of running castor oil in your fuel at a too lean setting (to hot).
Carbon build-up will always increase as you set the engine leaner...not richer!
A remedy is to use a mix of castor oil and synthetic oil or just all synthetic oil.
Over here in Europe many of us use 15% all synthetic oil ,or less,in our fuel.
(French racing brand: Motul Micro, or German, Graupner: Aerosynt 2 and Aerosave).
Regards!
Jan K
Sweden