As stated earlier I had a similar prob with my cb77 twin. All fine till it got into the higher rev range, particularly on the motorway. Like you, I was convinced it was the mixture. Played with the jetting for about a year even though I had bought new coils just in case. Wanted to keep it standard like but when I did remove the coils I found the problem side had leaked its oil into an almost solid goo. Sorted !
So try another coil, not expensive.