When an engine is operating at high speed and the throttle is suddenly closed to decelerate the automobile, a slightly rich mixture will be delivered by the carburetor. The combination of a closed throttle and high engine speed will create a high vacuum in the intake system downstream from the throttle plate. This will cause very little fuel flow into the carburetor throat but will cause a flow through the idle valve. This fuel, mixed with the low air flow rate, will give the rich mixture needed to keep good combustion. With the high vacuum in the intake system, a very large exhaust residual will occur during valve overlap, and a rich mixture is needed to sustain combustion. Misfires are common during this type of deceleration. Fuel injectors give much better A/F ratio control in fast deceleration.