Four stroke engine, in the power stroke, spark plug jump fire ignited combustible mixture gas, push the piston downward dead center movement. Because of the valve's "open early and close late", the piston opens the valve early before the dc on the power stroke. At this point, if there is an unfinished (that is, with a flame) mixture of gas into the muffler, continue to burn, will be fired.
    If, after the combustion, only high-temperature exhaust gas without flame is discharged into the muffler, the gun will not be fired. (there is, of course, another case where the ignition has just started up or the igniter has broken off, and the unburned mixture has entered the exhaust pipe, then the high-temperature exhaust gas can also ignite the mixture in the exhaust pipe and cause a shooting.)

     In other words, firing is related to the rate of flame burning and the time the exhaust valve opens.

     Let's first analyze the rate of combustion of the flame. This rate of combustion is most closely related to cylinder pressure and mixture concentration. The greater the cylinder pressure, the faster the flame burns. If the mixture is thicker, the combustion speed can be accelerated.

     As we all know, when measuring cylinder pressure with a cylinder manometer, the throttle valve needs to be opened more than half (full open is the standard operating requirement). The measured cylinder pressure is, for example, 900kpa(9 kg). However, if the throttle is idle, the measured cylinder pressure may be only 300kpa(3kg).

     When driving, suddenly loose throttle, throttle instant closure, the engine due to inertia, speed down slower.

     When the throttle is released, the cylinder pressure of the engine decreases in an instant, the suction path is closed, and the suction time remains as short as before. Causes the intake of mixed air to seriously decrease - the flame burning speed to slow down.

      As the engine speed drops slowly, the time of each working cycle is almost the same as before. Before the piston moves down from the top dead center of compression to the bottom dead center of power stroke, the exhaust valve opens almost the same time as before.

     So when the exhaust valve is opened, the unfinished mixture of gas with a flame rushes into the muffler and continues to burn, creating a shot.

     How to solve this problem? Some carburetors have an air cutoff valve that, in my opinion, is designed to relieve loose gas.

       At this point, the "air stop valve" on the carburetor is put into operation to thicken the mixture and increase the combustion speed of the flame. Allow the mixture to burn before the exhaust valve opens. In this way, only "hot exhaust gas" will enter the muffler, instead of the mixed gas with flame, and there will be no shooting.

     After the engine has slowed down completely, it takes longer for the piston to move down from the top dead center until the exhaust valve opens. On the other hand, during the intake stroke, the piston takes longer to breathe in, and the cylinder pressure is higher than when the throttle is released. So the air stop valve stops working and the muffler stops firing.

    Personally, from the perspective of fuel consumption, "air cut-off valve" can increase fuel consumption. But compared to "bang bang" blasting the muffler, this fuel consumption is acceptable.