Mikuni flat slide tuning guide: http://www.mikuni.com/fs-tuning_guide.html
Bingo!
Poor mid-range performance: " Flat throttle response in the mid-rpm range is seldom caused by either an over-rich or overly lean condition. Flat mid-rpm performance is more likely due to the effects of the cam or exhaust design. If the needle size is incorrect, it will normally reveal itself as poor mileage (too rich), slow warm-up (too lean) or light detonation when accelerating moderately from around 2500 to 2900 rpm (again, too lean)."
Yeah, I know... it's a Harley Mikuni flat slide.
Also...
http://www.mikuni.com/pdf/hsr_tuningmanual_021003.pdf
http://www.mikunioz.com/tuning-tips-and-manual-downloads/
http://www.iwt.com.au/mikunicarb.htm
edit also: And yes, it says never change main or pilot jets trying to fix a mid range problem. You can verify their performance, but they won't fix the mid-range. If you link to the correct carb on the mikunioz site, you'll find a tuning manual guide specific to your carb. Simple instructions.