Thank you all for the hints, I know my application has some limits (i.e. inverted engine and consequently a tad too high tank) but I hoped that the air bleed carb could have been adjusted with an acceptable compromise.
The carb barrel is clean, the valve lash is ok, the inlet pipe is leak proof. the plug is a Enya n°3 new.
So far I tried :
-without muffler pressure, with all air bleed open I still had a too rich setting ( it quitted at 3300 rpm, or sometimes during low-hi transition) high needle valve 2.5 turn open.
-With the tank pressure, I had the midrange too lean due to the high speed needle valve only 1 turn open, or the idle too rich in order to have a correct midrange .
So I have made a "T" connection among the pressure nipple, the tank vent and a calibrated orifice, in order to limit the max tank pressure,
playing with different orifice holes (1,5 mm, 1mm, 0,8mm, 0,5 mm)
I have at the moment the best result with 0,5mm HS needle open1&3/4 turns: I still have a too rich min, it dies after 30 seconds at 3000 rpm, but I have yet to adjust it better .
I tried to fly in these days to test it, but weather was tricky (rain, fog, or windy with low and dark clouds), and having the engine not running reliably cost me a regluing of the fin so far. So I am waiting better weather to have a final verdict, meantime I ordered a TN carb...
I will keep you informed, as soon as I will have updates,
thank you again
Paolo