Hi Wayne,
The Walbro WA-200 is most likely an 11mm venturi. There is no WA-540 since Walbro's highest WA is WA-215. The WT-540 is a 13.49mm and is used on many 42cc to 50cc chain saws. You can always tell the size of Walbro's by the number in 64ths cast into the venturi. The 13.49 is a common carb and a friend has a bunch for sale on RCU cheap. Here is the add: http://www.rcuniverse.com/market/item.cfm?itemId=106854

Dave