An adaptor flange for fitting under the mounting flange of a carburetor of an internal combustion engine and formed with a venturi type flow passage having perforated induction tube spanning thereacross for sensing the magnitude of the partial vacuum in such passage. A controller housing is connected on one end with an air filter and is formed with an induction throat leading to a control chamber. Disposed in the induction throat is a nozzle which is, in turn, connected with the carburetor float chamber. Mounted in the control chamber are first and second pressure responsive valves. Such valves are connected through air/fuel vapor lines to the induction tubes and are responsive to predetermined engine conditions, as sensed in said flow passage, to cooperate therewith in selectively varying the flow rate through said induction throat to correspondingly vary the mixture to the intake manifold.