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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.
 

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'83 VT750C
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Most of them already have a speedometer… 😉
The problem with ‘timing’, is these are basically closed systems. The engine was running fine before, the timing chain rarely slips/skips, if EVER, and there is no fancy computer advance/retard operations going on. Which leaves other electrical failure. Usually a bad battery, frayed or unplugged wire(s), or a bad ICM module or pulse generator that is breaking down with heat or stress.
Looking at the carburetor(s) is always a good idea, though. They can be so fiddly and finicky they can be out of balance for fuel delivery, which changes with gearing and higher engine loads.
So in cases like these, if the break-down happens at almost exactly ‘X’ rpm, in any gear, then it’s probably sensor or electrical. If throttle position related, or in higher gears, then it’s usually carbs or the fuel system.
 
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