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Let's see some photos of this bike!!
Flat-spot in mid-range is common on carbureted carbs due to helmholz-resonance effects. Air-pressure pulses move back & forth through carb, head, exhaust and frequency is determined by RPM, cam-specs, physical dimensions of carbs, intake-runners, exhaust pipe diameters & length, etc. Final effect is that in this RPM-range, you have a reversal of flow. This causes a triple-fueling effect where petrol is added to piece of air on its way in, then more petrol is added to it on way out, then yet again when it gets sucked in final time. Going to smaller carb increases air-velocity, changes resonance frequencies and reduces this triple-fueling effect.
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