Here's my web page about this that has a link to a free download of a spreadsheet that will tell you the carb diameter needed dependent on engine size, RPM, and intended use: http://www.dragonfly75.com/moto/carbsizing.html
The old formula many have referred to in the past is an over-simplification that none of the manufacturers went by.
dia=.8 x square root (cc x RPM/1000)
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www.dragonfly75.com/moto/ all about 2 Stroke technology, including calculators