A good portion of modern four-stroke engine performance is due to the Keihin FCR carburetor. An accelerator pump and a high-tech slide supported by four wheels make the FCR a relatively complex design. There are many parts that can wear out and get dirty or gummed up. There are a few shops that specialize in mods and service for these carburetors. Some, such as JDJetting (www.jdjetting.com; 253.939.7114) and Factory Pro (www.factorypro.com; 800.869.0497), sell kits that you install yourself, and others, such as Zip-Ty Racing (www.ziptyracing.com; 760.244.7028) and Tokyomods (www.tokyomods.com; 888.457.9403), will set up the carburetor for you or actually modify it for increased performance and response. White Brothers offers both options.As complex as the FCR carburetor is, it is more intimidating to look at than it is to work on. You will still need a good manual.Getting the carburetor off and back on will be the hardest part. You'll need to remove the cover on the side of the carburetor, disconnect the throttle cables, make sure the vent hoses are all free from the bike and disconnect the wire to the TPS. Do not remove the TPS from the carburetor; it is synched to the throttle shaft with a meter, and it must be resynched if removed. With perimeter-frame machines, it is almost always easier to take off the subframe and the shock before removing the carburetor.