Air-fuel ratio control system

In an air-fuel ratio control system for an internal combustion engine having a two-barrel carburetor comprising a primary side having a main metering system and a slow speed system and a secondary side having a main metering system, an induction passage, a throttle valve provided in each side, an exhaust passage, first detector means for detecting the concentration of a constituent of exhaust gases passing through said exhaust passage, two electromagnetic valves for correcting the air-fuel ratio of the air-fuel mixture supplied by said carburetor, electronic control circuit comprising a judging circuit for judging an output signal of said first detector means, and a driving circuit for driving said electro-magnetic valves in dependency on an ouput signal of said first detector means for controlling the air-fuel ratio to a value approximately equal to the stoichiometric air-fuel ratio;

• second detector means for producing an ouput signal when said carburetor changes from a primary side operation to the operation by both of the primary and secondary sides,
• a first passage connecting the slow speed system of said primary side to one of said electromagnetic valves;
• a second passage connecting the main metering system of said secondary side to said one of the electromagnetic valves;
• a third passage connecting said main metering system of said primary side to the other electromagnetic valve;
• an electromagnetically operated change-over valve for changing the connection between said electromagnetic valve and said first and second passages;
• said electromagnetically operated change-over valve being so arranged to normally communicate said electromagnetic valve with said first passage and to communicate said electromagnetic valve to said second passage by said output signal of said second detector means.