Hi, I notice you are using a Tillotson without the choke butterfly, same as a Kart carb. I ran these carbs on Lambrettas a few years ago, although the fuel pump and size is great I found they did not perform as well as a PHBL25 and the problem of cold starting was a pain if you didn't have the choke butterfly. If you ordered a tillotson with the choke butterfly then the venturi was limited to 22 or 23mm as I remember.
I even looked at making my own carb bodies that utilised Tillotson parts with PHBL choke parts and a much bigger venturi. However, when I contacted Tillotson about buying parts they were absolutely less than cooperative about the idea. I even looked at building the carb body into the inlet manifold to save costs.
You can buy a 24mm ventrui Tollotson from here
http://www.kartstore.co.uk/shop.cfm?cat=1505&node=129 which is a lot more cost effective.