A spark ignition internal combustion engine is described for a motor vehicle having an intake system comprising: an intake manifold (20), a main throttle (28) for regulating the rate of flow of ambient air into the intake manifold, a bypass pipe (30, 32) having a flow regulating valve (38) and arranged in parallel with the main throttle (38) to dilute the charge reaching the engine with additional air, an air flow meter (22) for measuring directly or indirectly the flow rate of air through the main throttle (28) only, and a fuel metering system (26) for supplying to the engine a quantity of fuel substantially stoichiometrically related to the measured air flow through the main throttle (28) only. The flow regulating valve (38) in the bypass pipe (30, 32) mechanically linked for movement in synchronism with the main throttle (28) and means (34, 35) are provided for selectively rendering the bypass pipe operative and inoperative so as to switch rapidly between two discrete values of air to fuel ratio in the total charge reaching the engine combustion chambers (10).