Is the secondary vacuum solenoid/servo working? The secondaries on Mikuni carbs are vac actuated - if the actuator fails, you lose everything from midrange throttle upwards. Easy to test. Take the small vacuum elbow off the actuator, add a longer hose and suck on it. If it doesn't hold vacuum, it's split (the secondaries won't open while the primary throttle is closed - just be aware of that). This unit can be be replaced without taking the carb off (some carbs have a rivetted shroud on them preventing access to assembly screws and auto choke fine adjustments) It takes a little prying and removal of a small 'C' clip on the secondary throttle shaft to get it out and reinstall.

*foot note - also check the vac advance on the distributor. They can bind up from gunk and the vac advance unit can also suffer the same fate as the secondary on the carb. This will nerf power/acceleration as well if it's inoperable