Spitfire Mixture problem

[reflected]

The Spitfire is supposed to have a 2 position mixture: normal or lean.
Both are adjusted automatically, lean is for high alt cruising. Am I not mistaken?

Well in game, especially high up the engine starts to lose RPM and smoke. So I set the mixture to "weak", but then it would totally lose RPM (I didn't exceed the +3 boost limit as stated in the manual) It happened at around 20,000 feet.

So is it bugged or what? How exactly does it work?

Thanks!