Abstract: A carburator for internal combustion engines has a main jet for delivering fuel to be entrained with air as it passes through the carburator to the inlet side of the engine. A throttle valve is disposed downstream of the main jet for controlling the flow of air and entrained fuel. An idling jet disposed downstream of the throttle valve is adapted to deliver fuel to the air stream at a rate sufficient to ensure that the engine continues running while the throttle valve is substantially closed. The idling jet is additionally adaptable to admit air to the stream of air-fuel mixture as it flows through the carburator at higher rates of air flow. Means are provided to switch the idling jet between fuel delivering and air delivering modes in response to engine speed. Thus, at lower engine speeds fuel is delivered through the idling jet and a slightly rich mixture passes to the engine. At higher speeds, air is admitted resulting in a weaker mixture than has been set by the main jet and the throttle valve.