My 1973 Honda CB450 sputters/bucks slightly at higher speeds (~50 mph). I get up to 50-55mph, then the bike starts sputtering/bucking a little, and slows down to like 40mph even if I lay on the throttle.
Very likely it is a points/condensors problem. Ignition breakdown at higher rpm's. One cylinder is not getting the proper spark. I would go with the simplest possible resolution first which would be to install new spark plugs. Carbs may need to be synched. I know this is not real specific but could be either, ignition or fuel related. These kind of problems cannot be correctly diagnosed by internet.
