Since I am using Maxima 927 Castor in the fuel, and know about the issue with the varnish and carbon when using castor oil. I might have a solution/suggestion.
I used to own an old 1979 100 hp Johnson outboard that used to carbon up a lot and foul a lot of plugs, every trip, it ran real cold. Mix was 50:1 synth pennzoil. I used to run stuff called OMC Carbon Guard (belive it was 1 oz per 8 gallons) in with the premix. Also every 100 hours or so I decarbonized using Bomadier Engine Tuner. Its is just an aerosol that you spray into the carbs with the engine at 2000 RPM , and it would really smoke and spit out black goo. Took one side of the head off and ....well you be the judge (see pics). I must say though it did not do much for the top of the piostons. I have heard people just spraying this stuff into the sparkplug hole and leaving over night and then the next day start it up and blow all the liquified carbon out. Had to change plugs after procedure. Might work for our engines too especially the ones that run rich or castor all the time. If you try this make sure air is flowed across the finsof the engine with a fan or better yet a leaf blower to prevent overheating. I'll try it after my next gallon. Let me know what you think.