I had the same problem with my X2. It would not have much in the bottom, but once it hit the midrange it would pull very hard to the top. I was running a 10/18 with a stock nozzle. I switched to an aftermarket PJS nozzle (+4mm) and it hit hard right off the bottom, but it would level out way too soon (too big of a nozzle). I was over proped and the ski needed to be up in the RPM and speed to hit. No adjustment to the carbs would change this.
You have to tune the package. Port timing, the pipe and the pipe tuning, prop, tail cone, nozzle diameter, they can all have an effect on the whole deal. The pump can matter a lot because excessive pump pressures can lock down the bottom end and it hits hard once the prop can finally spin. Port timing and your pimp affect the package a lot, and bad port timing or squish, etc can causes problems at certain RPM. The B-pipe for example can be tuned to hit hard in areas, or tuned to have a level power curve.
Fiddle with your carbs, if you can't tune it out, it is likely not a carb issue (assuming your carbs are working correctly). Try tuning your B-pipe, but if you level out the power there, you might lose it other places.
