@
Darksik -
As you are running a factory airbox, its safe to say you've got optimum air flow, pressures, etc....as dictated by airbox capacity and configuration to work in conjunction with the supplied CV carbs. I'd tend to trust the capabilities of those Kawasaki design engineers, tasked with meeting emissions standards, performance, fuel consumption and within economical goals. I'll go on record stating they did a pretty darn good job.
As Kawasaki sold a ton of 250's worldwide over a long production period....i'd further commend them with a corporate "mission accomplished!".
That said, you HAVE received valid info above by members @
Ralgha and @
DannoXYZ on needing to fully review fuel delivery and carburetor function and cleanliness.
As the airbox is (basically) a fixed entity...the fuel system (carbs) are, in fact, prone to negative influences.... restriction, maladjustment, weather, effects of poor fuel.... on and on.
Get the paper out of the airbox, properly clean and adjust them carbs and RIDE!