Hi Gang:

Got to assume this is not the official YS site, if it is, guess gonna get another butt chewing.
If you are ruinning pure castor, that a no-no with any current 2 or 4 stroke engine I am aware of. Should run a good blend of synthetic and castor oils. YS recommends generally 20%oil and 20% nitro. I'm on my 10-12th YS, never a running problem through the years and I abuse um. I use Power Master 15%nitro which has about 18% oils, motors seem to love the stuff. My worst screw up was recently sticking a after run oil recommended by supertiger, marvel mistry oil and ATF 50/50. into my YS's. They have been sitting for over 6 months and I was concerned they were getting dry. Well, I stuck the carburator barrels in two of the three. So now I am cleaning them out and replacilng all the gasgets, plunger and diaphrams and will use nothing but Performance Specialities after run oil called "Ultra Oil" in the future. Its also the lube they use in assbmbly of their engnes. Don't want to ever hear that "you dummy" laugh of D. Shadel again, even warranted. Oh well, another lesson. There is some great info on fuels in this forum, worth taking the time to read. Treat the YS engines reasonably well and they reward you with more power than the rest and years of reliable running. ENJOY