I have a new YS 120AC engine that was giving me trouble on the high end. Every flight was either too rich or too lean, and I had many engine failures. I was running Sig 15% nitro fuel with an 18% oil content. I bought 4 gallons total and kept trying each one to see if that was the problem. Nothing helped. Finally, I bought Wilcat 20% 4stroke fuel blended for the YS. The problem was fixed instantly.

I don't know if the nitro increase helped or if the Sig fuel had all come from a bad batch or .....

What I do know is that my problem is gone, and I thought this might help others.