A diaphragm-controlled carburetor which includes a diaphragm positioned in a chamber and dividing the chamber into wet and dry chamber sides. A valve is positioned in the wet chamber side, and is responsive to the diaphragm for feeding fuel to fuel jets and thence to a fuel/air passage. The dry chamber side is connected to receive pressure pulses from the engine crankcase through a three-position valve. The valve has orifices selectively interconnected by a rotatable valve disc for feeding pulses at first intensity to provide an enriched fuel/air mixture for starting, at a second lesser intensity to provide a less enriched fuel-air mixture for engine warm-up, and to block pressure pulses from the dry side of the control diaphragm.