Serge, sorry to hear of your frustrations. The screen in the carb is not the only place where debris can lodge. There are the jets on the other side of the carb that you cant see but can get stuff in them. Those jets are located under Welch plugs which would need to be removed and replaced after you inspected the jets. That would require a carb repair kit to get those parts.
I think your engine is new..??... have you removed from the plane.. mounted it in some kind of stand with different fuel lines and tank and tried running it that way?....did you open the LO speed needle to 2 turns open?..This would remove a Non Engine related problem specific to that plane/fuelsystem.

I am trying to save you the aggrivation of sending your engine to Southgate Aero. I'm sure its under warrenty if its new?.. I know Bill Oberdyke pretty well and they are good guys.. if you can, you can try what I suggested.. after that? you might give The Mfg a call to send it in.. Sorry I can't be more specific.. its the kind of problem that is simple once you know what it is.. its finding it that can cause fits!!... Regards BobH.