How do I Troubleshoot a Keihin Carb?by Don Davis
Keihin carburetors are constant velocity carburetors. They have a vacuum operated slide that changes the diameter of the main opening, called a venturi, in the side of the device. Constant velocity carburetors were invented by English motorcycle racers, but they are now called Keihin carbs after the Japanese manufacturer that first mass-produced them. For decades, these carburetors were standard equipment on numerous Japanese, British, German and American motorcycles, until they were eventually replaced by fuel injectors.
Turn on your motorcycle fuel supply. You will probably open a small faucet under the left side of your fuel tank called a petcock. The exact method of turning on the fuel supply may vary from motorcycle to motorcycle.
Pull out the choke. Pulling out the choke increases the amount of gasoline in the fuel/air mix.
Start the motorcycle in neutral and run the engine for about five minutes, until the engine begins to radiate heat.
Push the choke all the way in. If the engine stumbles or starts to stall, pull the choke part way out. If the engine idles with the choke pulled out -- but not with the choke pushed in -- your fuel/air mixture is too "lean," which means that the mix includes too much air and not enough gasoline.
Adjust the fuel/air mixture, by turning the low speed, idle adjustment screw on the bottom of the carburetor counterclockwise with a flat head screwdriver. Turn this adjustment screw no more than 1/8 of a turn at a time.
Twist the throttle open, with the bike in neutral, two or three times and let the throttle snap back. Keihin carburetors use two throttle cables and a spring-operated throttle return.
Listen to the engine. If the motorcycle runs without stumbling with the throttle open, but not at idle -- the idle speed may be set to low.
Identify the throttle cam on the carburetor by looking at the front of the carburetor and opening and closing the throttle. The sheet metal stamping that moves is the throttle cam.
Adjust the idling speed by turning the only screw on this cam -- it is called the throttle adjustment screw -- with a flat head screwdriver 1/8 turn at a time, until the engine idles without cough or stumbling.
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