CV Carburetor tuning

Proper fuel mixture tuning is the key to reliable starts, stable idle, and good performance. If your bike is properly tuned, it will start very easily. Gasoline can be a great fuel if it has the right amount of air to work with. Incorrect mixture manifests itself in many ways, some of which are not at all obvious.

Prior to any tuning, it is critical that the carburetor is completely clean. Follow the CV carburetor cleaning procedure first if the bike has been sitting for any significant amount of time or a dirty carburetor is suspected. There should be absolutely no vacuum leaks due to a worn stock vacuum petcock prior to performing this procedure.

Float Height
Proper float height is important to achieving proper air fuel mixture. With the carburetor turned upside down, the float seams should be parallel with the carburetor bowl seam. Proper float height can be checked externally using a clear tube connected to the float bowl drain. Refer to the diagram below:



Connect a clear tube to the float bowl drain, hold the bike level, and open the drain screw. With the fuel flowing into the carburetor, the fuel level in the tube will be the same as that of the float bowl. The fuel level should be slightly lower than the float bowl seam.

Idle mixture
Below is a cutaway diagram of the stock CV carburetor found on dual sport models:



The idle mixture screw is shown at point 9 above. From the factory, this screw is covered with a plug. The plug can be removed by carefully drilling a 1/8" pilot hole not more than 1/4" deep. An easy out or suitable sheet metal screw can be used to yank out the plug. Set the idle mixture crew approximately 1.5 to 2 turns out from a lightly seated position. The idle mixure screw can be replaced with an easy to adjust extended mixture screw. The extended screw is available from Kientech or ProCycle

Idle speeds of 1,400-1,600 RPM are known to work well. The idea is to set the butterfly open as little as possible while maintaining a stable idle.

Warm the engine up to operating temperature. Lower the idle with the idle thumb screw until the engine is about to stall. Then adjust the idle mixture screw to achieve an idle as high as possible. If the idle increases, lower the idle thumb screw again and repeat this process until the engine will no longer idle any higher by adjusting the fuel screw alone. If the mixture screw is significantly more than two turns out from a lightly seated position, you may need a larger pilot jet.

Set the idle between 1,400-1,600 RPM with the idle thumb screw to complete the procedure.