Bogdan, what is your engine and what fuel and glow plug are you using ?
By the way, in most airplane applications one can lean out the mixture for max power and smooth idle and it will give very good results, as plane engines are usually well cooled. On the other hand in most heli ( and also in RC cars ) the mixture setting is usually a compromise between power, engine temperature, avoiding detonation ( that can be seen as occasional tail kicking and metallic sound ), smooth running and so on.
Lots depends on the motor cooling solution in a particular helicopter - I guess cooling by fuel is more important on an average heli than for example on a Caliber 90 that has a special fan running at higher rpm than the motor - but otoh the Cali90 is often run in a closed fuselage ).
On YS motors I have tried different fuels and needle settings, only to go back to CP30% and to have the needle valve set by the book, with the high speed needle opened 45% more from factory settings ( that's due to the tuned muffler that I am using ). Also I have tried colder plugs, only to go back to Enya #3 again. Now the motor runs exceptionally smooth, idles well and in a hover the fuel in the tank looks like a glass of water on a table - no foaming, even very little ripples/fuel movements.
Too cold a plug will cause the engine to die too easily in idle,
also too lean mixture often results in high idle RPM after some flight time - if the motor takes several seconds to go down to idle then the motor temp could be too high or the idle needle too lean.
Btw. I do not set the idle RPM to be the lowest possible - but rather a bit higher from that, so the motor does not have tendency to die but still the clutch remains disengaged and the rotor does not move,