How to Tune a Mikuni CV Carburetorby Kelvin Hayes
The Mikuni CV carburetor can be tuned via 3 methods, the main jet that controls fuel flow during full acceleration, the needle clip position that controls fuel flow during cruising and roll on throttle, and the mixture screws that regulate fuel flow during idle conditions. The carburator should be tuned first by testing the motorcycle, and noting the characteristics of the idle, roll on and full acceleration. After testing you should start with the main jet, followed by the needle clip position and mixture screws.
Start the bike and note the idle characteristics. Does it idle smooth or rough? If it runs rough, the mixture screws will need to be adjusted. Test the roll on throttle to full acceleration to see if the motorcycle pulls hard at high RPMs. If the bike doesn't pull well under acceleration, this indicates that the main jet is too big and will need to be swapped for a smaller jet. If, during roll on acceleration, the bike pulls equally well whether warm or cold, this indicates the needle clip is set properly. If it pulls well while cold, the needle clip will need to be lowered, while the opposite is true for better warm pulling.
Loosen the hose clamp that attaches the carburetor to the air box. Loosen the hose clamp where the intake manifold meets the carburetor. Pull the carburetor loose and close the petcock valve on the bottom of the fuel tank. Remove the fuel inlet line from the carburetor by hand and pull the carburetor from the bike.
Unscrew the top of the carburetor, and allow the throttle, slide, spring and needle jet slip out of the top of the carburetor. This will allow you to adjust the clip position of the needle jet, but first you must remove the fuel bowl to access and adjust the main jet. Unscrew the fuel bowl bolts with a screwdriver and pull the fuel bowl away from the main body of the carburetor, ensuring to preserve the bowl gasket. Unscrew the main jet with a screwdriver and swap it for a smaller or larger size depending on the characteristics of your test. Screw in and tighten the new main jet and reinstall the fuel bowl, ensuring to place the bowl gasket between the bowl and main body to create a proper seal.
Spin the clip on the needle up for better roll on acceleration while the engine is cold or lower for better acceleration when the bike is fully warmed up. To start, adjust the clip one turn. You can always reopen the needle area of the carburetor to make additional clip adjustments. Slide the needle, slide, spring and throttle cable back into the top of the carburetor in one piece as it came out. Tighten the cap screws to secure the top of the carburetor to the main body. Slide the carburetor back into position within the bike, and slide the carburetor into the air box hose and intake manifold mount. Tighten the hose clamps on both sides to ensure the carburetor is secured to the intake system once again.
Start the bike and verify the adjustments have helped improve the performance of the bike. Repeat the steps above if performance is not improved, doing so until you achieve the results you want. Finally, adjust the mixture screw, found on the side of the carburetor body with a screwdriver to improve the idle conditions. Start with half turn increments while the bike idles until a smooth idle with adequate RPM to keep the bike running is achieved.