To automatically open a choke valve when an engine becomes set engine speed after the engine is started by arranging a switch at a set revolving number location of a lever revolving by interlocking with a governor mechanism and connecting this switch with a suction coil operating the choke valve to its opened side.

When an engine is started, a choke valve 2 of a carbureter is opened by a spring 23 at first. When cranking is performed by a coil starter, negative pressure of a venturi part 48 becomes large, large quantity of fuel is jetted from a main nozzle 5 and an engine is started. When the engine is started and engine speed rises, a governor lever 11 is rotated downward by a governor mechanism. A switch 31 is turned on by coming into contact with an abutting part 32 of the switch 31. As a result, current flows to a suction coil 25, an arm 21 is rotated downward against a spring 23, the choke valve 2 is opened and the engine is operated in air-fuel mixture of normal density.