stupid question about S-300C mixture lever

[Reely340]

AHA!

1. So how is that done?
2. Which direction for rich/lean?
3. When to turn that thing (only when all the way in or out, or anytime)?
4. Who is supposed to turn the control?
5. According to what behaviour/indicator/parameter ought it be set?
6. How can I check if the setting is off?

Especially 6. would be interesting, as various people starting the beast many times a year
might slightly alter the knob while doing so, unaware of its internal effects.

Background is I can lease two S-300C. One is really pulling well. The other one not so.
Said other one got a completely overhauled engine in 2012. Same lame power output.
This especially puzzled me as according to the mech "overhauled" means only the engine
case and crank stay, the rest is replaced. So with all new liners, rings, pistons, valves,
seats and camshafts the only reason for the noticeably worse performance might be
in the FI section (which to my knowledge is unchanged) setup.

TIA
Peter