Abstract: The invention is disclosed in the preferred embodiment as an electronic feedback carburetor system wherein, in the closed-loop mode of operation, an oxygen sensor monitors the oxygen concentration of the exhaust gases and supplies a signal to an electronic control unit which in turn causes a command signal to be supplied to the carburetor for adjusting the air/fuel ratio to a commanded value. The electronic control unit contains unique circuitry which selectively provides closed-loop and open-loop modes of operation depending upon the condition of other input signals to the electronic control unit. The circuitry contains integrator and stability circuits which are both utilized in the closed-loop mode to develop from the oxygen sensor signal a composite signal which provides for closely regulated control of the air/fuel ratio about a desired operating point at or in the vicinity of stoichiometric.