PURPOSE: To fully exhibit engine performance by increasing a fuel supply quantity with the atmosphere introduced in a carburettor float chamber from an atmosphere bypass passage when a throttle valve full admission signal, a signal showing an output- allowable set engine speed or more, and an electricity quantity of an output-allowable lean critical value or more are inputted.

CONSTITUTION: One end of an atmosphere bypass passage 70 opened to the atmosphere is connected to the midway of an outer vent passage 56, with which a canister 58 is communicated and connected, in the float chamber 36 of a carburettor 6, and an atmosphere bypass valve 72 and an orifice 74 are provided halfway the passage 70. The atmosphere bypass valve 72 is opened and controlled when a signal showing the full admission condition of a carburettor throttle valve 20 is inputted from a throttle sensor 46, a signal indicating the output-allowable engine speed (na) or more of an engine speed is inputted from a revolution sensor 50, and a signal giving the output-allowable lean critical value G or more of an air-fuel ratio is inputted from an air-fuel ratio sensor 44 respectively with a control means 48. This causes the action of the atmosphere on the float chamber 36 and the lowering motion of a float 38 to increase a fuel supply quantity.