Air intake side secondary air supply system for an internal combustion engine

An air intake side secondary air supply system for an internal combustion engine having a carburetor with a throttle valve, comprising:

• an air-fuel ratio detection means for detecting an air-fuel ratio from an oxygen concentration of an exhaust gas of the engine and for producing an air-fuel ratio signal;
• an air intake side secondary air passage leading to the carburetor on the downstream side of the throttle valve;
• an air control valve having a pressure chamber and disposed in said air intake side secondary air passage, for varying a sectional area of said air intake side secondary air passage in response to a magnitude of a fluid applied to said pressure chamber;
• a source of first control pressure for generating a first control pressure for opening said air control valve;
• a source of second control pressure for generating a second control pressure for closing said air control valve;
• and
• a pressure supply means for selectively supplying one of said first control pressure and said second control pressure to said pressure chamber in accordance with said air-fuel ratio signal, for gradually increasing and decreasing said sectional area, wherein said pressure supply means includes a first pressure supply passage and a second pressure supply passage, a surge tank provided in said first pressure supply passage, and said first pressure supply passage is a one-way pressure supply passage for directing only said first control pressure to said pressure chamber, and said second pressure passage is a one-way pressure supply passage for directing only said second control pressure.