Intake system of an internal combustion engine

An internal combustion engine having a combustion chamber equipped with an intake valve, an intake passage connected to the combustion chamber via the intake valve, a carburetor having a primary throttle valve arranged in said intake passage, a secondary throttle valve arranged in said intake passage at a position downstream of said primary throttle valve, means for opening said secondary throttle valve in accordance with a reduction in the level of vacuum in the portion of said intake passage which is located between said primary throttle valve and said secondary throttle valve, and an auxiliary passage having an outlet opening into the portion of said intake passage located downstream of said secondary throttle valve and an inlet communicating with the portion of said intake passage located upstream of said secondary throttle valve, wherein the improvement comprises:

• the cross-sectional area of a portion of the intake passage at the position of the secondary throttle valve is enlarged relative to the cross-sectional areas of the intake passage immediately upstream and immediately downstream of said enlarged passage portion, the amount of enlargement being sufficient to compensate for the flow resistance of said secondary throttle valve when said secondary throttle valve is in the fully open position, whereby the secondary throttle valve can divert the flow of intake gas into the auxiliary intake passage when the engine is operating under light load without reducing the volumetric efficiency when the engine is operating under heavy load;
• the intake passage further comprises a raised portion having a front wall formed on the bottom of said enlarged passage portion, the inlet of said auxiliary passage being formed in the front wall of said raised portion;
• and
• said secondary throttle valve has a cut away portion formed in the periphery thereof in alignment with the inlet of said auxiliary passage when said secondary throttle valve is in its fully closed position.