PURPOSE: To obtain a carburetor of good responsiveness by making a negative pressure introduction hole for a vacuum piston and a fuel supply line open and close through interlinking with opening and closing of a throttle valve mounted freely slidably on the outer periphery of a constant system type carburetor.

CONSTITUTION: A piston 21 to which a needle 35 is fixed is moved up and down by a diaphragm 22 due to a negative pressure from a hole 26. At the down stream side surface of the piston 21, a plate type throttle valve 18 is mounted, which can be moved up and down in response to the turning movement a lever 32 from the outside. At the opened time of the throttle valve 18, a groove 28 mounted on an upper portion side surface of the throttle valve 18 is opened to face a fuel nozzle 31 and the fuel is fed into an intake tube 2. When it is opened in wide range relatively to the piston 21, the hole 27 of piston 21 is opened and a much negative pressure is introduced into the upper side chamber of a diaphragm 22 and the piston 21 is opened. In case of a speedy acceleration which still can not be followed, the upper end portion of the throttle valve 18 is brought in contact with the end plate 34 of the diaphragm 22 and the throttle valve 18 is forcibly opened.