Can you change vernier sizes on Weber carbs, how do you measure them

Do you mean venturi?. They are marked with their size usually.

yours, Pete.

Yes Peter, i Venturi is correct, I have been offered a 32-36 carb but need to make sure that they are 23-24 mm for race regs , so can you alter themif so how mutch would it cost?

No you can't reduce the venturi size - however I could have just what u are looking for (it was sold to me on e-bay as a 32-36 but is actualy a 23-24) didn't cost me a lot and is useless for what I bought it for. As we have several already I'm not looking to make a profit more just get my money back - so could be a helpfull bargain? email me off group at dave@beddowsport.com if u are interested