Great!
Jim,
The all castor fuel may be gumming up the ring with time and causing inconsistant running. After break in, for ringed engines, I recommend a low percentage of castor (2%) this provides a measure of corrosion protection for the bearings and enough cushion for the rod and wrist pin and enough scuff protection for the cylinder/ring. The remaining oil (18%) should be a high quality synthetic such as that from DOW or Klotz. This recommendation is for any ringed engine including 4-strokes. Just a little castor for ringed engines adds a lot of benefit over an all synthetic. The high degree of synthetic will not gum up the ring and ring land in either 2 or 4 strokes. The small percentage of castor is'nt enough to gum up rings or valves. In any case with a ringed engine a higher percentage of castor is not a bad idea during break in. I would never consider using more than 4% castor (following break in) for long term use, gumming up of the ring could occur. I always recommend 20% total oil.
I'll be in Tucson on business for two weeks, privatly e-mail me and I'll give you the details and we can exchange numbers. I'll be on the south end of town at the base. See if any events, swap meets etc are going on.
Later on
Fuelman