Closure at last
Brief recap: engine started acting strange, leaning out at the top of outer loops or when flying inverted, even though the engine is side mounted.
I worked on if on and off for many months, until I ended up drilling another hole in the firewall for the fuel line. After rerouting the fuel line, all problems magically disappeared.
I think the problem was either the fuel or the pressure line pinching when subject to g forces.
Now the engine is very happy. I also switched from a MA 11x6 that I had before with a APC 12x4. Tached it at 13100 (Omega 10% nitro, 85F external temp, stock setup), with 6 hours on the clock.
Now I am thinking about removing shims, though I may just enjoy it for a while.
The Something Extra loves the new prop. Had to increase the tires from 2.5" to 3" to provide enough ground clearance.
Thanks for all who contributed ideas.