accelerated by the convergence of venturi throat 44 as the airstream passes the minimum diameter in venturi throat 44 it expands at a varying rate into exhaust port 46 in accordance with one aspect of the present invention a butterfly throttle valve 48 comprises a thin plate having apertures 50 58 therethrough apertures 50 are spaced to align with threaded apertures 52 in a throttle bar 54 to mount butterfly valve 48 thereon a throttle linkage system indicated schematically by linkage 56 rotates throttle bar 54 to change the angular position of butterfly valve 48 to selectively control the closing of mixing passage 40 thereby controlling the volume of air fuel mixture passing through carburetor 10 significantly apertures 58 permit the air fuel mixture to continue to flow through carburetor 10 when butterfly valve 48 closes off mixing passage 40 to provide a sufficient fuel supply to run the engine at idle the provision of apertures 58 eliminates the usual need for some other downstream port for supplying and independently sustaining fuel for idling fuel is supplied to carburetor 10 through fuel line 26 fig1 connected to a barbed connector 60 that is threadedly attached to a fuel input port 62 a fuel volume adjust screw 64 is threadedly engaged with carburetor body 38 and is adjusted to limit the maximum amount of fuel which can be supplied to carburetor 10 fuel volume adjust screw 64 includes a tapered end 66 that extends into blocking relationship with fuel input port 62 as more clearly shown in fig3 in order to provide for precision control of the fuel volume permitted.