I bought a used ns 200 sometime ago and this is one of the more things wrong with it. I can’t hold 2k or 3k no matter how lightly I twist the throttle. Carb is cleaned although with air pressure and not fuel. Jets are not blocked. And throttle spring and cable is good. Diaphragm and slide has been replaced. There was a vacuum leak in the carb boot but has been replaced. One more trouble is that the rpm comes down slowly after a blip. It troubles me to tell that the bike is running on castrol edge.