To ensure start of an engine at low temp. while retaining a throttle valve at the increasing position of air and fuel with a simple operation.

A level 11 transmitting an accelerator action to a throttle valve 4 is turned from idle position by means of a cam piece 22 and is simultaneously moved to the direction of axial line so that the degree of overrupping of a throttle through hole 5 with an intake passage 2 is made slightly large, and the insertion depth of a measurement needle 7 into a fuel nozzle 15 is made slightly low, thereby increasing the quantity of air and fuel which are required for the start-up at low temp. The cam piece 22 is engaged with a lever 11 and fixed at a working position, and is returned to unworking position by a spring 23 when the lever 11 is disengaged therefrom by the accelerator action.