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.

Just have to butt in here the ys motors must have at least 20% nitro to run like they were designed to run i have used sig fuel also and they just dont run right . The sig oil is very thick viscosity and dosnt flow correctly , glad you found the fix, i have been using either Cool Power heli 30% or the Wildcat 20/20 and have had good results with both.