Modern Vespa

its impossible, ive tried every mixertube with 140 ac and 160, either its to lean or to rich. atleast with the airbox on.

Have you checked your motor for air leaks? Jetting a bike without ruling that out is impossible.
Provide some more information and I'm sure you can get the help you need.

I feel your pain. I have worked on dozens of Vespas and sometimes the SI carbs just will not work right no matter what you do. I spent days checking everything possible on them and I could find nothing wrong but they still didn't work. They are effective but relatively crude.

Sometimes I get lucky with a carb swap and everything works fine. Why a specific NICE used carb or even some NEW Spacos won't work is unclear.

For example - I had a VBB that I rebuilt the engine on and it wouldn't run right no matter what I did. I bought a rusted out parts bike that a farmer had outside in a ditch for decades. I pulled the original crusty carb off it and threw it on my rebuilt engine and it ran great. I didn't even clean it! WTF?

Other times I would have to switch to a different style carb altogether. Side draft Mikunis where my favorite. Not because they are easy to tune (they aren't) but because they a very precise and allow you to dial the mixture in very accurately.

This isn't helpful but its what I have experienced. Sometimes you have to get creative and throw out the book.

What is your engine setup and what jet stack(s) have you tried? Also what poor running symptoms are you getting?