DIRT BIKE STUNT

Tech How-To
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.

01 Ty Davis starts by thoroughly cleaning the FCR body in a solvent tank filled with Simple Green rather than using a petroleum-based solvent or contact cleaner. He warns that you should never spray contact cleaner into the mouth of a carb.	02 If you have an aftermarket fuel screw installed, this is a good time to remove it. Take care so you don't lose the small spring, washer or O-ring. If you still have the stock fuel screw, removing it can wait.	03 You will need to extract this bolt from the throttle linkage above the slide. It may be a secure Torx with a tiny post in the middle. You may need to visit a special tool store to find a dimpled Torx socket.
04 With the Torx removed, the slide's lifting arm can rotate out of the way, and the slide can be pulled out. The brass fitting on top of the slide holds the needle in place.	05 The slide has a wear plate that lifts off. Some companies recommend changing it every 25 hours. It is an expensive part, but replacing it can make a big difference in how crisply and cleanly the bike runs down low.	06 The back side of the wear plate has this very delicate seal. Davis warns that chemicals like contact cleaner will cause the seal to swell and ruin it. It is a good idea to replace the seal when the carb is apart.
07 Davis removes all the vent hoses at this point. Small brackets mounted on each rear corner of the float bowl guide some of the vent hoses. Remove the bowl screws and the idle adjustment bracket.	08 Work off the float bowl and look for any dirt or other junk in the bowl. This one is quite clean. Most water and sediment will settle in the drain bolt.	09 With the bolt removed, the throttle pivot shaft should simply pull out. Note the return spring attachment and the placement of the two washers. Clean up the shaft and assorted parts. Set these parts safely aside.
10 At the top of the slide will be some arrangement that allows removal of the needle. Some off-road bikes have nonadjustable needles (only one clip position), and they can be replaced with an adjustable one for tuning.	11 Using a small tool, push out the float pin. A small Allen wrench or the back end of a drill bit should work fine. Once the pin is moved, you should be able to pull it out with your fingers.	12 The float manipulates this float needle to keep the fuel level in the float bowl constant. Look for damage near the pointed rubber tip. The needle is loosely attached to the float via the wire loop. Don't lose it.
13 The main jet is threaded into the needle jet, so remove them together. The fuel baffle (called a "spacer" in the manual) will come off easily after the needle jet tube is out.	14 Unscrew the starter jet and the pilot jet. A blade screwdriver with a tip or shaft larger than 4mm wide will jam in the pilot-jet opening. Clean all the jets, and ensure that all of the openings are unrestricted.	15 There is a screen at the bottom of the needle-valve seat that prevents trash in the fuel from entering the float bowl. It cannot be removed for cleaning, so use compressed air to blow it clean from this side.
16 Use a finger to operate the accelerator-pump rod through a full stroke. You should feel spring resistance but no hard spots or hitches in the travel. If it doesn't move freely, pull it apart and clean it.	17 A small screw holds the fuel-line fitting in place. The carburetor was otherwise quite clean, but this fitting and passage need a bath.	18 A little grease on the O-rings will ease the cleaned fitting back into the carburetor body. Don't get carried away with the grease. Install the screw that holds the fuel-line fitting. It shouldn't need thread-locker.