This system provides economical mixture ratios for part throttle operation while still permitting richer mixture ratios for full load operation. During most of part throttle range of operation a “back suction” on the fuel in the fuel bowl is created by manifold vacuum, through the channels connecting the throttle bore with the fuel bowl. This retards the flow of fuel through the metering systems and thus permits the carburetor to operate on leaner mixture ratios at part throttle. Rotation of the throttle shaft controls the economizer system. During part throttle operation (from one-quarter to three-quarters throttle), the passages are open and the pressure in the fuel bowl is lowered which retards the flow through the main jet to supply a leaner mixture. At full open throttle, the passages are closed and the main jet flows to full capacity to supply the richer mixture required.