A 38mm is the right size for a motocross bike, all for top RPM power. An IT is rarely using top RPM power and so needs a smaller carb, something from 28mm to 32mm, to emphasize power at mid range RPM.
But no matter what carb you use there is a set sequence you need to follow to find the right jet sizes and the right tapered needle: www.dragonfly75.com/moto/carbtuning.html
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