A tip.
PROBLEM SOLVED!
It was to hight friction between "inner" and "outer" trottle valve arm. I had no control where
#1 carburetor trottle valv was positioned. A little oil and exercise solved the problem...
Thanks for your advices.
Now I have removed the critical bolts, none snapped, and reinstalled them with Loctite LB 8009.
Hope they comes off easily next time I need to unscrew them.