Carburator problems - please help!!

I recently broke my timing belt and went on to replace it, along with valves and the whole works (while we were at it) once we finally received the parts from Piaggio Spain (by the way, I live in Madrid, Spain).

After mounting the whole thing back, we noticed we gained a lot of compression, but also, some problems between 3.500 - 4.500 rpm with the carburator! Before and after that, it works fine - but its sputters a lot in that range.

My mechanic has gone mad with it and it´s 4 different settings and seems to be at a stoping point with it - he can´t get any parts or decent help with Piaggio since it is a discontinued model and doesn´t know what else to try. Eventually, he´ll need gaskets, but we are considering a new carb or a replacement for it in order to have the bike running correctly again.

Has anyone ever suffered problems like that?
Is it true that yamaha´s XT600 carb is the same?
Where could I get parts/new carb?
Has anyone any idea of what I could try?

I am running desperate after a whole month and a half with out my bike...

Thanx in advance for any help/coments on the matter