Just found this on the "Carburetor Overview (Keihin)"
Quote:
Air-Cut Valve
An air-cut valve is used to prevent popping in the exhaust system during deceleration. The valve enriches the air/fuel mixture during deceleration. A diaphragm in the air cut valve is activated whenever high manifold vacuum is present, such as during deceleration. The movement of the diaphragm causes a partial blockage of the air bleed system in the low speed circuit. This reduces the aeration to the low speed jet which creates a richer mixture. The air-cut valve is factory pre-set and should not be altered. It may have to be replaced eventually, because the rubber diaphragm in the valve may deteriorate over time. If the diaphragm is deteriorating, you may notice leanness or a popping in the exhaust during deceleration.
