The little number in the barrel of the carb is 64th of an inch. Figure that out in decimals of an inch and convert to metric to get the size in millimeters. I never understood why the just did not put 13.49, 15, 16.6 or whatever, in millimeters in the first place.

For instance, I am holding a WT542, the choke side has a little "34" inside. 34/64= .53 .53 x 2.54 = 1.34 so 13.4 mm.