I think you are probably measuring the wrong part of the carb. What you want to measure is the smallest part of the carb throat which is called the "venturi".

You want one of two Walbro carbs for your engine:
Look just inside the entrance of the carb near the choke. It will have a number cast into the carb throat. You want either a 28 or a 32.

The 28 is an 11.1 mm venturi.

The 32 is a 12.7 mm venturi.

AV8TOR