Carburetor Tuning, adjusting the idle mixture

The first step in tuning the pilot circuit is, or any, carburetor is to get the idle circuit correctly adjusted. That means adjusting the idle mixture screw for the best idle. Keihin carburetors sets the idle mixture screw during assembly and seals the screw under aluminum plug to prevent an owner from changing the setting.

If the aluminum plug is removed, which can be done by carefully drilling a small hole in the plug, threading in a small screw, and then pulling out the plug, gently turn the adjusting screw all the way in until it bottoms. Then turn it two and a half turns out from the fully closed position.

Next, start the engine and bring it up to operating temperature. With the bike in the vertical position and the idle near 1000rpm, turn the idle mixture screw in slowly until the idle either slows or becomes irregular. Now turn the screw out until the engine again slows or begins an irregular idle, counting the number of turns between the too rich and too lean positions.

Then set the idle mixture screw midway between the two positions. You can refine this setting later if you want, but this will be very close to the perfect idle mixture setting.

As you continue tuning the carburetor, the mixture setting is going to change if you change anything else in the carburetor. So, if you end up replacing the main jet and changing or shimming the needle, you'll need to re-tune the idle circuit. Luckily, the idle circuit is the easy part and is relatively easy to accomplish.

For adjusting the mixture screw we can only say, practice and patience is the key here. Note: If the best idle is achieved with the idle mixture screw less than a quarter turn out, the pilot jet is too large and will need to be changed to a smaller, leaner one. If the idle mixture screw is more than three turns out, the pilot jet is too small and will need to be changed to a larger richer one. Disclaimer: We accept no responsibility for drilling holes in your carburetor, and be aware that by drilling a hole in your carburetor you most likely not have warranty on your carburetor And we also aware that carburetors are called carb's or carby in the USA, even sometimes called chiefly in Australia, and I belief some even write carburetor