System for controlling an air-fuel ratio

In a system for controlling the air-fuel ratio for a carburetor of an internal combustion engine having an induction passage, a throttle valve in said induction passage, an exhaust passage, detecting means for detecting the concentration of a constituent of exhaust gases passing through said exhaust passage and providing a detecting output signal dependent thereon, air-fuel mixture supply means for supplying the air-fuel mixture to the induction passage, including an on-off type electromagnetic valve for correcting the air-fuel ratio of the air-fuel mixture supplied by said air-fuel mixture supply means, the improvement comprising

• a comparing circuit means for comparing the detecting output signal of said detecting means with a reference value corresponding to a stoichiometric air-fuel ratio value and for producing a first output signal dependent on the difference,
• an integration circuit means responsive to said first output signal for producing an integration output signal,
• a driving circuit means for producing a driving output for driving said type on-off type electromagnetic valve dependent on the integration output signal of said integration circuit means for controlling the air-fuel ratio to a value approximately equal to the stoichiometric air-fuel ratio,
• a constant signal generating circuit means for producing a constant signal selectively operating said on-off type electromagnetic valve at a predetermined duty ratio via said driving circuit means,
• a first switch connecting said integration circuit means with said driving circuit means,
• a second switch connecting said constant signal generating circuit means with said driving circuit means,
• means comprising a vacuum switch in said induction passage for sensing only such a high vacuum occurring in a rapid deceleration condition of said engine that would cause misfiring, and
• a switch actuating circuit means responsive to the output of said vacuum switch for producing a second output signal for opening said first switch and closing said second switch,
• said first and second switches are operatively connected to said second output signal of said switch actuating circuit means, and
• inverting means connected between said second output signal of said switch actuating circuit means and one of said switches for opening one of said switches while the other of said switches remains closed and vice versa, respectively, via said second output signal, whereby said driving circuit means is operated by the constant signal from said constant signal generating circuit means for driving said on-off type electromagnetic valve at said predetermined duty ratio.