to which a fuel air mixture is supplied through an intake manifold 3 from an air cleaner 4 and a carburetor 5 after combustion exhaust gases are discharged from the chambers 2 through an exhaust manifold 6 into the surrounding atmosphere the carburetor 5 consists of a carburetor barrel or housing 13 extending downwardly from the air cleaner and forming a fuel air mixing chamber or passage 7 within the fuel air mixing chamber 7 a choke valve 8 mounted on a shaft 12 controls the flow of air received from the air cleaner below or downstream from the choke valve 8 is a venturi 9 with a nozzle 10 located at the venturi for discharging fuel into the chamber a throttle valve 11 is positioned below the venturi 9 for controlling the flow rate of the fuel air mixture to the combustion chambers the shaft 12 on which the choke valve is mounted is supported in the housing 13 and extends outwardly from one side of the housing as shown schematically a casing 14 is mounted on one side of the housing 13 and the shaft 12 extends into the casing and is connected to one end of a bimetal spring 15 supported within the casing the rotary shaft 12 for the choke valve 8 is connected to the.