I'd head for the usual suspect on the Mikuni carb - the secondary vacuum can (the round vacuum actuator on the side of the carb that is attached to the secondary throttle shaft linage. With the engine off and the air filter removed, disconnect the small vacuum hose that is connected to it (a small elbow shaped hose). Connect a longer vacuum hose into the top vacuum barb on the actuator and open the throttle all the way, then apply vacuum to the attached hose. If it pulls the secondary linkage open and holds the vacuum then it's serviceable. Normally the diaphragm inside the vac can splits, causing an air bypass and killing mid to top range power.