It is true that mixture becomes richer at high altitudes (logically because the air becomes thinner) therefore a higher fuel to air ratio, but some aircraft had an automatic compensation system which meant the mixture was left in the rich setting.

MadTommy, the hurricane works perfectly indeed (and some other planes too), but not the Spitfire (I haven't tried the 109 with CEM yet).

Is it that the Spit I has got only two positions for mixture and prop pitch indeed, instead of a bug?

I'm not a 109 flyer, but doesn't it have an automatic control of prop pitch keeping RPM in the optimal zone?

On these (IRL) Hurricanes and Spitfires the Mixture controls were 2 Position not fully variable.
So think of them as "Auto Rich" or "Auto Lean" that you find on many aeroplanes. So in Lean for example you are in effect just saying I want to run in Auto Lean then using this as a Datum the mixture is automatically leaned as altitude is increased and enriched as altitude decreases. There is no need to be manually adjusting mixture as you climb. In general Weak (Auto Lean) is only going to be used when you want max range or endurance, in Combat you want RICH. Typically there are boost limitations associated with particular mixture selections

In addition the RICH position is at the REAR and WEAK position FORWARD. Also the quadrant was so designed that if the Throttle is brought back to idle it will also drag the mixture lever BACK to its RICH position. You can see this in operation on the Second selectable Hurricane.

The First selectable Hurricane is bugged as the Throttle slides ghost like through the Mixture lever. in the case of the Spitfires its bugged as the Throttle does not drag the mixture lever back. Devs are aware of these bugs

Some of the "rough" running we see is the engine being way to sensitive to the -ve G cut. At present a controlled slow (like around 0.8G) nose lowering has the engine coughing. This sensitivity needs to be fixed so normal smooth attitude changes can be accomplished without the coughing we now see with the slightest lowering of the nose.

I think that depends on the model. And from what I see of the E3, it is manual prop pitch control.

Unless there is a key to be mapped for auto somwhere?

Auto Prop pitch on the E model 109 didnt come into service effectively until Nov 1940.

Is it me or does the Mixture actually work in reverse to the cockpit animation? For example I can't get enough power from the engine at takeoff in the First Solo training sortie unless I move the mixture lever animation to Lean (Forward). Also, at altitude it runs rough unless I move it to Rich (should be lean).

And yes, on some Merlins there were two mixture positions not incremental: Auto Rich and Auto Lean (or was it just Lean?)

btw, on the Prop Pitch, there were various applications, this from the Spitfire I and II pilots notes in the A2A FSX Sim (can't find my Actual notes atm):

STARTING THE ENGINE AND WARMING UP:
1. Set mixture control fully rearward to NORMAL (RICH)
2a. If de HAVILLAND 2-POSITION ADJ AIRSCREW COARSE PITCH (plunger in) then,
immediately after start to FINE PITCH (plunger out)
2b. If ROTOL CONSTANT-SPEED AIRSCREW FINE PITCH (lever forward)
3. Set radiator shutter lever full forward FULLY OPEN
4. Raise both fuel cock levers to ON (only one cock in the Hurricane, click on the red end)
5. Open throttle slightly on quadrant OPEN ¼”
6. Unscrew primer (counter clockwise) <<<<<<<<apparently not enabled in CoD?
7. Inject petrol shots 5 AT NORMAL TEMPERATURE <<<<<<<<apparently not enabled in CoD?
8. Ensure everyone is clear of the airscrew “CLEAR”
9. Switch both magnetos (in Mk I, also the starting magneto) to ON <<<Starting Mag not enabled in CoD?
10. Switch Generator to: ON <<<<<<<<apparently not enabled in CoD?
11. Throttle CRACKED
12. Hold START button and prime until engine catches <<<< Primer (plunger) not enabled in CoD?
13. Throttle OPEN only as far as avoids rough running
800 – 1,000R.P.M. (BY EAR)
In Mk I - Switch starting magneto OFF <<<Starting Mag not enabled in CoD?
14. Screw down priming pump (clockwise) LOCKED <<<<<<<<apparently not enabled in CoD?
15. See that the oil pressure is satisfactory AT LEAST 45 lb./ sq. in.
If not, SHUT DOWN ENGINE IMMEDIATELY and report condition to maintenance officer.
16. Warm up at fast tick over (1,200 R.P.M.) until oil temp is 15C
Mk I - radiator temp is - 70C
Mk II - “ - 60C

Engine testing blah blah blah........

11a. If WEYBRIDGE FIXED PITCH AIRSCREW: no R.P.M. check is necessary. Open up to full
throttle momentarily and check boost is as expected.
11b. If de HAVILLAND 2-POSITION AIRSCREW: Check R.P.M. 2,500 – 2,600 at FINE PITCH and check for positive R.P.M. drop at COARSE PITCH
11c. If ROTOL CONSTANT-SPEED AIRSCREW: Slowly reduce airscrew control to 2400 R.P.M.
Throttle down a little and observe R.P.M. to be CONSTANT at 2,400 R.P.M.
Then, set airscrew control lever fully forward to: FULL FINE PITCH

Required reading for all CEM noobs (like me)
 

found this really helpfull

Anyone knows whether the cockpit animation in Spitfire Ia in regard to the mixture lever has been corrected in the latest patch? Or is it still rich in the forward position?

I don't think this has been done yet. Open an info window (Alt+Right Click) and select Engine Info. The Mixture output will be shown when you move the mixture lever. 100% is Rich, 0% is Lean.