Sorry to Hi-Jack this. its not a carb, engine or fuel/air issue. its spark timing. I have the same problem ish. bike begins stumbling/ missing bad during fast rpm rate change in first and second gear. usually WOT or close to it. pull perfect to about 4k then starts missing bad and barely accelerates. I have a tac that works correctly installed on my 07 sabre 1100 and the bike used to run perfect until redline around 6k.. My hope is that someone on here has some real experience and knowledge to offer other than take your carbs apart. if it was a fuel/air issue it wouldnt have a definitive rpm that initiates the problem. gravity fed carb engine. spark advance/timing is the only possibility. if im wrong someone please drop a knowledge bomb and scold me. PS what is with almost no one on this forum running a tac. a tac is also a speedometer if you can do 3rd grade math.