There are 3 components that get overlooked when rebuilding the Mikuni - the secondary vac actuator, the auto choke release assist and the wax pellet for the auto choke. Reason being is that these 3 parts are expensive and don't always need replacing. They will fail without much of a warning. The wax pellets are a difficult part to find and are a biatch to take out and install. Although the Mikuni is a relatively 'good' carb, it's a complex carb to strip down and reassemble, can be expensive to overhaul and is designed primarily to meet emissions standards and are not easy to adjust. For performance - nada. These carbs seriously strangle the engines they're bolted up to. If you aren't restrained by emissions inspections, a Weber 32/36 (which was the old stand-by) or a 38 DGES will make a world of difference. Not a straight swap and requires trial and error to tune, but when set up correctly will noticeably improve economy and performance.