The actual measurement is going to be at the throttle valve, then an honest seller will tell you both the throttle valve diameter as well as the venturi diameter... I've seen these 14.5mm carbs measured to the outer diameter of the spigot mounts, the daimeter of the air filter flange... Yup... I've seen one actually claimed to be a 72mm carb... for these engines.
Bottom line is the engine can only take in what the smallest diameter of the carb is so if the carb is 14mm at the throat but has a 12mm venturi, it's a 12mm carb and will perform accordingly. The standard NT carb is 14mm and the NT speed carb is 14.5mm. MOST your Dellorto SHA clones are also 14mm at the throttle and have a 14mm venturi, but they also make the 14.12 which has the 14mm throttle opening and a 12mm venturi.
The only accurate way to measure anyof these carbs is by the throttle valve opening diameter and the venturi size, and if the venturi is smaller than the throttle opening then that's the size that needs to be gone by.
Be real careful buying off ebay and some of the other vendors as they tend to measure the wrong parts of the carb, either because they simply don't know any better or they're trying to pass theirs off as being bigger than they really are. I've seen some that are advertised at 19 or 20mm then look at the pics where they have the dial caliper on the outside of the spigot or the largest point on the inside of the clamp on the clamp mounted carbs, which basically leaves the customer either thinking it's truly a 20mm carb or totally unable to know the true size of the carb since they can see the smaller opening but no measurement is being taken at the most important areas.
Here's a prime example of one that's obviously measured at the wrong points...
http://www.bikeberry.com/engine-kit-parts/performance/66-80cc-high-performance-carburetor.html What they're probably meaning is that is has a .78 jet installed and the air filter flange is 59mm, but they probably don't know to measure the carb at the venturi, I really don't think these guys are trying to lie about it's size, but it's more of a case of their staff members not knowing where to measure these as what's really important is the throat and venturi diameter.
