Well, I am running Power Master 15% right now. I have been running McCoy MC9 Plugs also. I didn't have this problem while I was running some 5 year old Nelson 30% (I had to tell the age cause I don't even know if they are still in business). The Nelson was stored in a cool dark place for those 5 years while I played with fixed wings.

I only got this PM 15 because HP had a $9.95 sale over a holiday. I usually mix my own but can't mix it for that cheap. I just mixed up some of my 25/20 today and will try it tomorrow. Maybe that could be the answer. I plan to also try the OS #8 plug but only want to change one thing at a time so I know what resulted in what.

Yes, My OS 50 has that little "half turn" mixture screw. They don't like to call it an Idle screw and rightfully so, since it adjusts the midrange. It is a relatively new engine with less than 8 gallons. If I try to lean the top end to solve that fat midrange, I cook the engine in FFF.

I agree with your practice of tuning the High End according to your last flight. Too many people like to "tweak" before EVERY flight and that is just asking for trouble.

Dan