Abstract: An internal combustion engine having a helically-shaped intake port. When the intake manifold vacuum is in the range of about -500 through -250 mmHg, a super lean mixture is fed into the cylinders of the engine. When the intake manifold vacuum is in the range of about -250 through -100 mmHg, a fuel mixture, having the stoichiometric air-fuel ratio, is fed into the cylinders of the engine. When the intake manifold vacuum is smaller than about -100 mmHg, a rich mixture is fed into the cylinders of the engine.

Abstract: An engine includes a three way catalytic converter in its exhaust system, and a carburetor whose throttle valve has a normal and an idle up idling position, selected between by an idle up device. A device for weakening the air/fuel ratio of the mixture in the engine intake system only functions when no oxygen is detected in the engine exhaust system by an oxygen sensor. A means for supplying enough secondary air into the exhaust system upstream of the oxygen sensor to make the exhaust gases leaner than stoichiometric functions according to supply of a controlling vacuum. A first vacuum take out port is provided in the throat of the carburetor at a position downstream of the throttle valve when it is in the normal idling position and upstream of it when it is opened a little from the normal idling position.