Carb Issues

First of all Hi to you all,

This is my first thread so here goes....

I have recently bought a 250N Superdream on a `T` plate.
Unfortunately its not running properly which is why I think the guy sold it.
The carbs fitted look like they are off of a 400N, I say this because on the side of the carb it has an `air cut off valve`
(I dont think the carbs on a 250N does. according to my Haynes manual, I might be wrong )
but curiously, the carbs have VB30BBSF stamped on the side of them ?

The Jets fitted in the carbs are 105`s ?

What is the correct standard jet sizes for a 250N ?
and
A 400N ?

Can anybody explain whats right / wrong ?

Thanks