PURPOSE: To prevent over-rotation of engine at the time of startup by controlling a solenoid valve to open or close the path of mixed gas for startup, bypassing a throttle valve, by not only temperature but also revolving number of the engine.

CONSTITUTION: A fuel path 7 is connected to an air path 6 bypassing the body 2 of a carburetor to form a supply path for mixed gas for startup, and the mixed gas is supplied through a solenoid valve 3 from a nozzle 12. The solenoid valve 3 is opened when the temperature 13 of engine is detected to be low, but closed when revolving number is increased by putting revolving number signals from an ignition coil 14 into a controller 16. The set value of revolving number is varied by signals of an engine temperature detector 13 to keep the revolving number proper.