As Minnflyer said, raising the nose at full throttle is a better way to test high speed needle settings. (In my opinion.) And if it's dieing in the air it's probably too lean.

As YNot said, you should not be playing with the needle(s) all day. Once set, I seldom touch my needles from one week to the next. The only time it's neccessary is if the weather changes substantially. Like a LOT more hot and humid, or it gets much colder.

All engines will develop a dark brown reidue over time. It's the unburnt castor oil cooking on the engine, and it's natural. If it's developing quickly (after a few flights), it could be a sign of running too lean however.
Dennis-