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It's all going to depend on the type of carb, carb bore, jetting, etc etc.
As an example -
My bike stock uses a 40 mm CV carburetor. Throttle response is rather sluggish, and jetting for best mpg reveals a bog just off idle.
Many guys swap to a 39, 40, or 42 mm carb with an accelerator pump. The biggest reason if for quicker throttle response, but what I've been finding in my research is that mpg increases are made as well.
Stock, the bike is good for about 55 mpg
Tweaking jetting for noticeable gains in throttle response knocks that down to the high 40's.
Jetting for best throttle response means a very overly rich mixture to compensate for the lack of accelerator pump, and many guys are stuck struggling to even get 35 mpg.
In comes the pumper carbs, and now you can jet for a steady cruise, and tweak on the accelerator pump for the throttle response you are looking for. Many report being right back up over 50 mpg with great throttle response once dialed in.
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