Abstract: A down-drafting constant vacuum pressure type diaphragm carburettor includes a carburettor body with an induction passage, a throttle valve, a Venturi portion variable of open area. A vacuum actuated valve is arranged at upstream side of the throttle valve across the induction passage and extending in horizontal direction from one side to the other side, and controls an open area of a Venturi portion depending upon a vacuum pressure generated in the Venturi portion. A jet needle is mounted on the vacuum actuated valve. A fuel control system including a diaphragm arranged between a recessed portion of the carburettor body and a cover covering for defining a regulator chamber at the side of the recessed portion and an atmospheric pressure chamber at the side of the cover, and a valve seat. The diaphragm is arranged at lower side of a longitudinal axis of the jet needle.