In an acceleration pump of a carburetor for an internal combustion engine including a cylinder and a piston reciprocatorily inserted in the cylinder, the piston is formed in its upper portion with a transverse bore for a stopper member to extend therethrough to be urged by a heat sensitive element to move in a direction at right angles to the direction of reciprocatory movement of the piston, so that additional fuel for acceleration can be supplied to the engine in a quantity which is accurately corrected to accommodate itself to changes in ambient temperature, by means of the heat sensitive element. The transverse bore is formed at its top wall and bottom wall with projections, and the stopper member is formed at its upper surface and lower surface with depressed portions, so as to enable the lengths of the downward and upward strokes of the piston to be controlled in accordance with changes in ambient temperature. Thus the acceleration pump is capable of supplying to the engine additional fuel for acceleration in a quantity which is corrected to accommodate itself for changes in ambient temperature.