Abstract: A downdraft type carburetor having a conventional idle speed air/fuel mixture channel has an air/fuel mixing chamber that mixes the idle channel air/fuel mixture with idle bypass air from the main induction passage, the mixing chamber being connected to discharge into the induction passage below the throttle valve through a nozzle containing an orifice sized with respect to a larger orifice in the air bypass passage to provide a pressure differential creating sonic flow, the nozzle extending so that the sonic flow is followed by a shock wave, the turbulence created atomizing the fuel.