Carburetor having air fuel ratio adjusting means

A carburetor system for an internal combustion engine, comprising:

• an intake air passage leading to an intake port of the engine and having a venturi portion,
• a float chamber containing fuel maintained at a predetermined fuel level,
• main fuel supplying means comprising a main fuel discharge nozzle centered in the venturi portion and a main fuel passage leading from said float chamber to said main nozzle,
• air bleed means comprising an air bleed pipe inserted into said main fuel passage,
• auxiliary air introduction passage means for introducing air from the outside to said main fuel passage, said auxiliary air introduction passage means comprising a primary air passage having an upstream end communicating with the outside and a downstream end opening into said main fuel passage below a fuel level in said main fuel passage and a secondary air passage which branches off from said primary air passage at an intermediate position between the upstream end and the downstream end and opens into said main fuel passage at a predetermined height above the fuel level, said secondary air passage having a cross sectional area smaller than that of said primary air passage, and
• valve means connected with the upstream end of said primary air passage for regulating an air flow introduced into said primary air passage.