PURPOSE: To prevent the quick-opening of a secondary valve when a choke valve is operated, in the case of an apparatus in which the secondary valve of a two-stage carburettor is operated by the negative pressure of a primary venturi, by providing, on the negative pressure passage, a valve which causes the negative pressure to be lowered at the time of low temperatures.

CONSTITUTION: In a two-stage carburettor 1 provided with a primary throttling valve 8 and a secondary throttling valve 9, the secondary valve 9 leads, through a passage 14, the negative pressure in the negative pressure port of a primary venturi 4 to a diaphragm 11, and when the negative pressure is larger than a fixed value, it is detected that the intake air quantity is in the increased state, and the secondary valve 9 is caused to open. On the passage 14 is provided a valve 16 which opens and closes in response to the temperatures of cooling water, and at the same time, are provided a passage 14B from the negative pressure port of small caliber, and then, the passage 14A is selected at the time of low temperatures when a choke valve 10 is closed. Hereby, even if the choke valve 10 is operated to cause a large negative pressure to be produced on the downstream side from the valve, in virtue of the pressure from the negative pressure port of small caliber, the secondary valve 9 will not make quick- opening.