The operation of the pump circuit is quite similar to that described for the vacuum operated metering rod; however, the movement of the diaphragm, instead of positioning the metering rod, is used to pump a small quantity of fuel from the bowl into the carburetor bore. When the throttle is opened, intake manifold vacuum drops, the spring forces the diaphragm forward, closing the intake check valve and forces a charge of fuel through the discharge check and out of the pump jet into the bore of the carburetor. When the throttle is closed, the intake manifold vacuum increases and the diaphragm is pulled back. This action closes the discharge check valve and draws a supply of fuel from the bowl into the pump via the open intake check valve.