PURPOSE: To prevent useless fuel supply during no-firing rotation, and to eliminate lack of initial power by providing a by-pass communicating an upper stream part of a carbureter with a lower stream part in an inlet air passage connected to an inlet air port of an engine, and by providing an opening and closing valve which opens during a running period, and closes during a stationary period.

CONSTITUTION: A by-pass 18 is provided in an inlet air passage 7 of an engine 3 on both sides of a carbureter 9 so as to communicate an upper stream passage 7a to a lower stream passage 7b. An electromagnetic valve 19 is provided at the middle of the by-pass 18, and in addition, a chamber 20 is provided in the lower stream part of the electromagnetic valve 19, and further, an exit port 18a for the lower stream passage 7b is connected to the position just before an inlet port 10. This electromagnetic valve 19 is closed during a stationary period. So, the pressure in the lower stream passage 7b coincides with that in the upper stream passage 7a. Thus, even if the engine 3 rotates in a state of no-firing, fuel supply to a fuel chamber 11 is prevented. Lack of power at the start is prevented by the fuel remained in the lower stream passage 7b.