PURPOSE: To ensure a proper air-fuel ratio upon high speed operation even if the air/fuel ratio is set rich upon trolling operation by providing an air communication hole in which air jet is connected at its inlet, in the side direction of intake passage of a carbureter, enabling the hinge shaft of a throttle valve to penetrate the air communication hole laterally, and providing an opening and closing hole in the hinge shaft.

CONSTITUTION: An air communication hole 5 is bored in the side direction of an intake passage 2 in parallel therewith. At the opening of the air communication hole, air jet unit 6 is installed. The hinge shaft 3 of a throttle valve 4 penetrates the hole 5 laterally and is provided with an opening and closing hole 7 at the penetration position. At the throttle opening when an engine speed is at 200rpm or less, the opening and closing hole 7 does not correspond with the air communication hole 5 and air is interrupted, so that the air/fuel ratio is set rich, thereby ensuring stable trolling operation. When the engine speed is raised by setting the opening of the throttle valve 4 to the level over the trolling operation, the opening and closing hole 7 corresponds with the air communication hole and a specified amount of air flows from the air jet unit 6, whereby the air/fuel ratio is compensated properly.