Constant Velocity Carburetor
Close to the engine side of the carburetor, there is a spring-loaded butterfly valve that rotates on a shaft, connected to the throttle cables. There is a stop screw that prevents the valve from completely closing. The small open airway supplies air to the engine at idle. When the throttle is fully opened up the butterfly valve is parallel to the airflow, offering minimum resistance. This section of the carburetor is of large diameter to prevent the butterfly and its shaft from reducing airflow. At the edge of this butterfly in its nearly closed position are tiny holes that supply fuel and air for idle, and off idle operation. Fuel for these passages is supplied through a pilot or idle jet, which is screwed into the bottom of the idle fuel passage, submerged in fuel in the fuel bowl. Air to these passages is regulated by the idle-mixture screw or air screw.
Just upstream from the butterfly valve in a smaller diameter section of the carburetor is a vacuum-controlled throttle piston. At its lowest position this piston doesn't completely close off the carburetor bore. This piston is attached to a needle which as it rises allows more fuel to enter the carburetor.
Reason For CV Carburetors[edit | edit source]
Fuel is 600 times heavier than air by volume, and if the rider directly controlled the throttle piston, and jerked it upwards, air would rush into the engine, leaving behind the heavier fuel. This would result in a temporarily lean condition, resulting in misfiring and lugging. Thus in CV carburetors the throttle is only attached directly to the butterfly valve, allowing for the vacuum piston to rise on its own all while picking up the correct amount of fuel from the needle hole.