Sounds like its a tad rich, I had this on the lower cylinder when i was playing with the 300 on my 18. That would stubble and cough between 5k and 8k.. then clear and go like the clappers. In the end i ended up fitted power jets and running smaller mains, this gave me more control over the mid range fueling with the needle.

One way to test it.. find a hill.. ride up it with it at the point where its coughing and kill it. Check the plugs.. no doubt one of them will be wet and black.
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If I have to take the carbs off once more...