PURPOSE: To avoid a bad influence on a catalyst by a usual ignition cut and also control an engine rotation speed in good responsiveness in engine rotation speed control structure.

CONSTITUTION: In an engine provided with a carburetor 1 for supplying a fuel stored in a float chamber 12 in an intake passage 2 by the suction out by the negative pressure of a ventury part 2c, the float chamber 12 of the carburetor 1 is communicated to either one of a place 9 in which the negative pressure is generated by the engine operation or in the atmosphere through a valve 22 in a free switching manner.