I,m surprised no one has hit on your engine problem as of yet. Especially here on rcu, but anyways your problem sounds like it's with the float diaphragm lever that controls the needle and seat. It is not getting enough fuel at full throttle because the float diaphram needle is'nt opening up enough. The diaphram needle sits in the cavity right under the side plate with the hole in it. The diaphram itself which has a metal pin in the middle, pulses the lever and needle to control fuel flow. If the lever is pushed in to low then the needle will not rise enough to allow full fuel flow at WOT.

I had 3 ZDZ carb problems just like this and each one I had to adjust the diaphragm lever to get enough flow. Once this was done the High end needle went from 3 turns open to get a slightly rich setting to 1 1/2 open for a really rich setting.

There is a guy named Xipp on Flying cirkus that just explained (in a post) about the workings of a walbro carb. I think you will find this very useful
[link]http://www.flyingcirkus.com/forum/tm.aspx?m=51010[/link]