PURPOSE: To raise the combustion limit of a dil. mixture, by providing an auxiliary intake passage which is separate from a main intake passage and has a smaller diameter than the main intake passage and by causing a swirl in a cylinder.
CONSTITUTION: An auxiliary intake passage 16 is provided separately from a main intake passage 7. The diameter and cross-sectional area of the auxiliary intake passage are made smaller than those of the main intake passage. The outlet port of the auxiliary intake passage 16 is opened just behind an intake valve 14 provided at an opening of the main intake passage 7. The inlet port of the auxiliary intake passage is coupled to a place which is in the upstream of the throttle valve 4 of a carburetor 2 and has high air pressure. The outlet port of the auxiliary intake passage 16 communicates as an injection port with a pipe 17 driven into a cylinder head 6. Turbulent flow is caused in a cylinder by a jet introduced from the auxiliary intake passage. This results in burning a thin mixture well.