It sounds like you have not fried a gas engine yet. They can run too lean and damage themselves just the same as a glow engine.

Back to the original problem of running erratic running while inverted there is one more thing to try. Get a couple of helpers that you trust and fire up the plane inverted. If it is still erratic it is most likely a mixture problem. If it runs smoothly then it is still an inflight airflow problem affecting the carb.