It's very difficult to diagnose running problems on a bike until all the various maintenance tasks are up to date. I'd start with all the basics as detailed in the factory Kawasaki service manual, and make sure all the factory parts are up to snuff. Valve adjustment, good spark plugs, clean air filter, airbox fully sealed, mufflers not rotted and falling off, and lastly, the carbs need to be 100% clean and with fresh O-rings to assure proper sealing.
A lot of guys buy aftermarket "carb kits" when the time comes to clean their carbs but most of the time this takes you down the wrong path. For one thing the original Mikuni jets are very high quality and you should try to reuse them unless damaged. Aftermarket jets are often of questionable quality. Taking the carbs fully apart, soaking everything in carb dip followed by blowing out all the passages, and then lastly reassembling with new O-rings and gaskets will most likely solve the carb issues.
Good luck