I agree with Falcon269 comments although water is more inclined to be a slow starting issues than a high end rev issue unless you have a lot of it, but it could be an accumulation of dirt in the bowl which is being sucked up under hard acceleration, so draining as he said may solve the issue but for a different reason. If it improves after draining you may have to remove the carbs and clean out the bowls.
also check to ensure the fuel pipes are all ok, no kinks check both the one from the tank to the filter and the pump to the carburettors.
Just because the air box inlet is clear is not enough to check, you will have to lift off the air filter cover to ensure nothing was sucked in.
Would not be the first time a little furry friend made a nest in there.