PURPOSE: To obtain a stable fuel supply during acceleration and such fuel discharge characteristics as are suitable for cold and warm operations.

CONSTITUTION: A level 15 for imparting a stroke to the piston 8 of an accelerating pump 7 is connected through a spring 21 to a primary throttle valve 3. There is provided midway of the stroke of the lever 15 such operation delay means 25 as is operative to impart the delaying action to the operation of the lever 15 when a preset opening is reached by the valve 3 thereby to prevent the excessively fuel enriching phenomena in the abrupt accelerating range and the excessively lean phenomena in the transient region to secondary. The mechanism thus far described is equipped with a temperature-sensitive controller 35 which is made responsive to the engine temperature to control the delay means 25 in an interchangeable manner between the operation blocked and released condition. Thus, it is possible to obtain such fuel discharge characteristics as are suitable for the cold and warm operations.