PURPOSE: To perform an optimum air-fuel ratio control of an engine by a method wherein actual measured values regarding the rpm of the engine and load data are compared to reference values to thereby determine a corrected duty of the engine and the corrected duty is further corrected in the power region of the engine by adding a predetermined value thereto.
CONSTITUTION: The actual values regarding the rpm of the engine, the negative sucking air pressure and the sucking air temperature are measured at Step 122 and the measured values are compared to prestored reference values so as to determine and then correct the duty of the engine at Steps 124 and 126. Then, when it is found by detection that the corrected duty is not for the power region of the engine, a control solenoid is controlled on the base of the corrected duty through steps 136 and 138. On the contrary, when the corrected duty is found to be for the power region, the secondary valve of a carburetor is inspected if it is opened or closed and when opened, a correcting values S is added to the corrected duty while when closed, a correcting value P is added to the corrected duty. The results such additions of the correcting values are transmitted to Step 136.