I had an issue for about two weeks, that had a similar symptom ( no power from idle, to midrange. At high rpm, both cylinders running fine.)
My spring in the carb slide would actually get bent/stuck sideways. Preventing the slide to open, until a higher vacuum was achieved to release it. I found this out because when I opened the carbs, one spring was stuck sideways.
Cutting out the long story of testing, and resetting...repeat..
I replaced the set of springs, and the issue hasnt come back.
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