Air-fuel ratio controlling system for internal combustion engine

An air-fuel ratio control system for an internal combustion engine, comprising:

• a carburetor including a main fuel system which opens into a venturi portion of an air intake path;
• a slow fuel system which opens into the intake path at a location downstream from the venturi portion, a main bleed air passage permitting communication between a portion of the intake passage upstream of the venturi portion and the main fuel system;
• a main bleed air control passage connected at its downstream end to the main fuel system;
• a slow bleed air passage permitting communication between a portion of the intake passage upstream of the venturi portion and the slow fuel system, and a slow bleed air control passage connected at its downstream end to the slow fuel system;
• a valve for controlling the amount of bleed air supplied through the main bleed air control passage and the slow bleed air control passage;
• an oxygen sensor for detecting an oxygen component in an exhaust gas flowing through an exhaust passage;
• and
• an electronic control unit capable of carrying out a feed-back control for controlling the operation of the valve on the basis of a signal from the oxygen sensor to bring the air-fuel ratio to a theoretical air-fuel ratio, the electronic control unit determining a control value in such a manner that a feed-back control value is learned and stored to provide a learned value which is substantially constant with respect to fuel of a constant calorific value, wherein the stored learned value is reflected to a next control value.