A carburetor includes a passageway having a constricted section, a nozzle directed into the passageway proximate the constricted section, and a shaft having a surface that at least partially defines the constricted section. The nozzle is configured to deliver fuel to air passing through the passageway, and the surface includes a contour that is configured to be moved relative to the passageway to change the area of the passageway through the constricted section.