The accelerator pump system provides a measured amount of fuel which is necessary for smooth engine operation during acceleration. Accelerator pump action is controlled both mechanically and by manifold vacuum in the same manner as the metering rod. When the throttle is closed, the diaphragm moves downward and fuel is drawn into the pump fuel chamber. When the diaphragm moves downward, the discharge check is seated. When the throttle is opened, the diaphragm moves upward, forcing fuel out through the discharge passage, past the discharge check, and out of the pump jet. If the throttle is opened suddenly, the pump upper spring will compress, resulting in a smoother pump discharge of longer duration. Manifold vacuum is applied to the underside of the diaphragm at all times when the engine is in operation. When manifold vacuum decreases to the point where the pump lower diaphragm spring overcomes the manifold vacuum, the diaphragm moves upward and a pump discharge results.