Modern Vespa

because the fuel feed is gravity fed driven there will be ALWAYS a pressure difference following from the height of fuel weighing down on the tap opening.
The only solution to a constant pressure is an electric fuel pump.

There is a breather hole above the float chamber. When you turn the carb upside down, this is where the fuel runs out from.

You quite right. Absolutely "d'oh" moment. Yesterday I take off carb for cleaning. All was clean inside only very little dirt on the filter.
But...
Here you is little bastrard.

Interesting that when I cleaned carb two times before there was nothing. I went to work today on PK and almost everything was fine.

This problem remains, but now it is not very noticeably - now it appears after 800-1000 m on WOT. I'm absolutely can live with it because I very rarely ride PK in the city on such high speeds.
But I will try to shorten fuel hose because it have loop than I don't like and next step will be change fuel tap.

Normally thing kind of thing is a hair or piece of plastic but that will do.

For the other issue are you sure the main jet is big enough? Works like rough or jerky are often used when talking about lean jetting.

It's impossible to tell from your picture, but is there any point where there's and overhand loop and you could get an air bubble?

Thanks. I will try bigger main jet. Thats what I'm thinking about. It seems that there is place where loop on hose is higher than fuel tap on the tank.