Intake system of an internal combustion engine of a counter-flow type

A multi-cylinder internal combustion engine having a plurality of cylinders each having a combustion chamber, an intake port, an intake valve, and an exhaust port, said engine comprising:

• an intake manifold comprising a collecting portion having an inlet, and a plurality of manifold branches branched off from said collecting portion, each of said manifold branches being connected to the intake port of a corresponding one of the cylinders;
• a carburetor having a throttle valve and arranged in the inlet of said collecting portion;
• at least one auxiliary intake passage, common to at least two cylinders and having an inlet which opens into said collecting portion and an outlet;
• at least two auxiliary branch intake passages branched off from the outlet of said auxiliary intake passage, each of said auxiliary branch intake passages having an outlet which opens into the intake port of a respective one of said at least two cylinders;
• and
• secondary valve means arranged in each of said manifold branches and opened in accordance with a reduction in the level of vacuum in said intake manifold.