To make an exhaust condition and controllability of an engine good by easily and precisely correcting errors of a fuel flow rate due to dispersion of components of a carburetor.

An emission passage 23 of a fuel pump 15 is connected with an intake passage 20 by a bypass passage 27 and one part of the emission fuel is released to the intake side in order that the pressure acting on a fuel valve 10 of emission fuel transferred to a constant fuel chamber 5 from a fuel pump 15 acting by pressure pulsation is not more than a defined value, a manual regulating valve 29 steplessly regulating the passage area is installed to the bypass passage 27. Changing a relief volume of the emission fuel makes opening of the fuel valve 10 easy or hard, so that fuel retaining volume in the constant fuel chamber 5 is regulated so as to obtain a predetermined fuel flow rate.