YS 63 Chattering Sound
Bad fuel will be hard to needle. In fact, it will be impossible at idle and mid range. At high speed, the needle will act funny. The engine will sputter like the ignition is bad. You'll think it's rich. So you lean it out more. Then you're too lean.
I just ran a Y-S 91 with some bad Heli 30 yesterday. Couldn't set the low end to be perfect to save my neck. I then tried the fuel in a Magnum FS120 and it ran the same way. I had some Omega 10% setting around and they tuned right in.
Just because you just opened the box, doesn't mean that the fuel is fresh? Try some in another engine? The humidity in Enterprise, Ala must be close to 100% this year. Maybe it has water in it?
Enjoy,
Jim