Today I staretd my mvvs 26 gas: after about 45 minutes of break-in I tryed to open to full trottle(propeller meijnik 18/8) but after 5500 rpm the engine dyes. Hig screw too lean? I used to feed the engine a bottle near the model with a tube to the carb.

forgot to say that I did not change the facory settigs on carb

Paolo, look here for [link=http://www.prme.nl/forum/viewtopic.php?t=10]complete setting of the walbro carb[/link].
Also check the filter inside the carb, and clean it. You can see it when you remove the cast carb cover (1 screw only)

thanks for the answer:how can I know if the engine is rich or lean, if opening the trottle after 5500 rpm the engine tend to stop that means that is lean? if I can fully open the trottle and can't get 8000 rpm(18/8 prop and stock pipe) it means that is rich?

It depends,
if the situation improves with the needle more open, it was lean, and vice versa.
Just move the needle, and find the best rpm. Then open a bit more so you are sure the engine will not be running lean. That will be the most powerful setting, and the rpm you get will be what you get.