I recently swapped engines on my 1981 cb250 to a cb400n engine and swapped overthe carbys as well as I assumed the jetting between the 250 and 400 carbys would be different. The 400 seems to be running very rich as it has a backfire when coming off throttle and misses when not under load (misses when cruising at low speed and not accellerating). I didnt change over the cdi unit, would this be different? Im wondering if this would be a jetting issue, blocked jets, or the carby would need a overhaul. Also if someone could confirm wether the jets are the only different bits between the carbys. I might put the 400 jets into the 250 carbys and see how that goes. Any input and ideas would be appreciated. Cheers.