Engine air-fuel ration control method with venturi type fuel supply device and fuel control appliance including the method

An air-fuel ratio control method of an engine with a venturi type fuel supply device comprising, at least, a venturi chamber located in the upstream of a throttle valve and a passage for supplying air-fuel mixture gas into the venturi chamber;

• wherein
• the passage is equipped with a variable air bleeder valve for taking in air, and when the operating state of an external load of the engine changes, the opening of the air bleeder valve is adjusted in accordance with the change so as to control the air-fuel mixture ratio of the mixture gas incoming from the passage into the venturi chamber two or more control variables are provided for adjusting the air bleeder valve opening in accordance with the change in the operating state of the external load, and the opening of the air bleeder valve is adjusted by switching the two or more control variables, a valve opening transition processing is provided for adjusting the air bleeder valve opening, thereby the opening is adjusted gradually, and the transition quantity and the transition time of the air bleeder valve on the occasion of switching from “there is no external load” to “there is an external load” condition are set differently from those on the occasion of switching from “there is an external load” to “there is no external load” condition.