Multicylinder internal combustion engine

A multicylinder internal combustion engine comprising cylinders to which rich air-fuel mixture is supplied, cylinders to which lean air-fuel mixture is supplied, a rich mixture intake manifold supplying mixture from a rich mixture forming device to said rich mixture cylinders, a lean mixture intake manifold supplying mixture from a lean mixture forming device to said lean mixture cylinders, a manifold connecting port through which said two manifolds communicate with each other, an open-and-close valve installed in said connecting port, a pressure-responsive device that has a first and a second movable diaphragms vertically spaced in casing and comprises a first chamber defined by said first movable diaphragm and casing and a second chamber defined by said second movable diaphragm and casing, a rod that connects said first movable diaphragm to said open-and-close valve to actuate the valve, a mixed vacuum passage through which a mixture of vacuum in a first vacuum passage communicating with an intake passage upstream of a throttle valve in the perfectly closed position and vacuum in a second vacuum passage communicating with an intake passage downstream of the throttle valve in the perfectly closed position, a third vacuum passage connecting said second chamber to at least either of said rich and lean mixture intake manifolds, a conduit connecting said first chamber to said mixed vacuum passage, a first spring that always urges said open-and-close valve toward the closing position, and a second spring that always urges said open-and-close valve toward the opening position with greater force than said first spring.