Abstract: An apparatus for regulating the quantity of air-fuel mixture supplied to each of the cylinders of an internal combustion engine and for selectively obstructing the flow of the air-fuel mixture to one or more such cylinders to effectively disable those cylinders. A butterfly valve is provided for each cylinder of the engine and is positioned in the passage in the intake assembly which supplies the air-fuel mixture to that cylinder at a location adjacent the end of the passage which is remote from the carburetor. The valves are controlled individually or in groups by a manually operated control mechanism. In one embodiment, the intake assembly includes a valve plate which is positioned between the engine block assembly and the intake manifold, with a portion of each branch passage being defined by a respective opening through the valve plate and each butterfly valve being provided in a respective one of such openings in the valve plate.