08 Ninja 250 struggles to stay running below 3k rpm

I bought my nephew a Ninja 250 that had been sitting for a few years. Took the carb off to check it out and they were coated in green residue, so I bought a whole new carb.

After cleaning the tank, installing the new carb, and putting in new fuel lines, the bike dies when taking off under 3k rpm and has almost no power below 5k

Any thoughts?