On the Walbro carbs, look into the carb throat near the choke butterfly. There is a number cast into the aluminum. That number is the venturi size in 64ths of an inch.

28 = 28/64" = .437" = 11.1 mm
32 = 32/64" = .500" = 12.7 mm
34 = 34/64" = .531" = 13.5 mm

(To get decimal numbers, divide the top part of the fraction by the bottom number. To get millimeters, multiply your decimal number by 25.4)

AV8TOR