Digitally controlled bypass shut-off system

This invention provides a digitally controlled bypass shut-off system to be implemented in two-stroke or four-stroke engines, comprising: a capacitor discharge ignition system, a carburetor, an engine rotation speed detector, an air bypass valve and a computer controlled unit. The carburetor includes a throttle valve position detector with an upstream that is connected to the air bypass valve or a piston-type air bypass valve. The computer controlled unit makes decisions based on the signals generated by the throttle valve position detector and the engine rotation speed detector, and outputs a signal to control the air bypass valve so as to obstruct the intake air from entering the carburetor thereby shutting off fuel that enters the engine to prevent fuel waste, and to control the optimum ignition timing, to obtain the optimum torque, and to avoid engine explosion, so as to improve the combustion state of the engine and to conserve consumption of fuel.