PURPOSE: To permit to open the throttle valve in two steps with single actuator by a method wherein two sets of operating rods, holding the throttle valve at respectively specified degrees of opening, are provided in series and two steps to introduce suction vacuum into respective vacuum chambers through a control means.

CONSTITUTION: The actuator 6 for opening the throttle valve 4 of a carburettor in two steps is provided with a diaphragm 8 and first and second vacuum chambers 14, 16 defined by first and second diaphragms 10, 12 provided at both sides of the diaphragm 8. Respective diaphragms 10, 12 are provided with the operating rods 22, 24 coaxially and are energized outwardly by springs 18, 20. The vacuum chamber 14 is communicated with a vacuum port 28 through a vacuum path 30 provided with a vacuum switching valve 40, opened when a starter switch 44 is put ON, and a temperature sensitive valve 38 opened when the temperature is higher than a predetermined engine temperature while the vacuum chamber 16 is communicated with the vacuum port 28 through the vacuum path 32 provided with the vacuum switching valve 46 opened when an electric load switch 50 is put ON.