In addition to Mike:
A slight leaking of the internal fuel needle will cause fuel admittance to the jets. With the cylinder hanging down, this fuel drips to the lowest point, causing problems. With inverted engine, NEVER move the prop against compression, as this will set the carb pumping!!!!!
Also, set the needles. The brilelli setting is a moment in time at the brilelli workshop. As the engine gets time, settings change. Your altitude may differ like this, following air oxigen content:
higher = leaner, or Vise versa
Hotter = leaner, or Vise versa
Air Moisture higher (humidity)= leaner, or Vise versa.

If your engine runs better in cold dry weather, it is too rich!