Dear Pe,
Last weekend we were working in the bench with one third engine after breaking-in. We started with L needle 45 minutes and H needle 50 minutes. the engine performed perfectly right side up burning 1 liter of gas. When we put engine up side down, it was doubting and lasting to reach the maximum rpm when we pulled the throttle so we closed the L needle 5 minutes because at half throttle the engine seemed rich, one of the two cylinders were burning too much gas and running in four stroke. after closing only 5 minutes the L needle the engine run perfectly with fast acceleration. we wasted another liter of gas.
But when put the engine right side up, again it quit when WOT, just at the end.

We tried shortening the stroke of the sick even losing 100 rpm, using 40% exponential curve, using a curve by points but all unsuccessfully. the only thing that seemed to work was closing partially the hole of the carburetor with a finger.

Summarizing, up side down, L needle needs to be closed 3-5 minutes (aprox. 40 minutes total), right side up L needle needs 3-5 minutes opened, all with H needle 50 minutes opened. There is no common point that works both sides, there is a commitment between both sides, if you want to run perfect right side up, inverted runs rough.

I am wondering whether a filter in the intake of the carburetor will help because we realized that putting a finger in the intake, closing slightly it in order to avoid entering a lot of air suddenly, it worked.
Could you be so kind to let me know your suggestions about the filter?

Is it possible that the carbon fiber sheets inside the carburetor need some breaking in in the sense to get them much more flexible. I am asking myself it those parts are working properly when they are new.

Thanks a lot for your help.
Best regards,