An air-fuel ratio control system for an internal combustion engine includes an air intake connected to a burning chamber having an oxygen sensor and an intake manifold to an engine combustion chamber. Fuel is mixed with air in both the air intake and the intake manifold. The oxygen sensor maintains a 14.5/1 air-fuel ratio in the mixture in the air intake, and a pressure differential sensor equalizes the pressures in the air intake and the intake manifold. The rate of flow of fuel into the intake manifold is varied so that the engine operates at a high economy, low emission point when idling, running at a substantially constant speed or decelerating and operates at a low economy point, yet still with low emissions, only when accelerating.