Abstract: A decompressor disposed to start an engine by a recoil starter and at this time compression pressure within a cylinder is adapted to escape therefrom, in which a decompression valve of the decompressor is disposed vertically with respect to a crank shaft or within a crank casing, so that a decompression valve does not laterally project from the cylinder and does not interfere with a cowling, whereby a compact engine is laid out, and also an interlocking mechanism of the decompression valve with the recoil starter becomes simple. Furthermore, a cam at the recoil reel is made variable correspondingly to the kind of engine so that the decompression timing during the starting is made variable so as to obtain the most suitable starting mechanism.

Abstract: Air is drawn into the crank chamber when the negative pressure is built up in the crank chamber, and the air mixed with the blowby drawn into the crank chamber is led as the secondary air to the exhaust gas discharge section when the positive pressure is built up in the crank chamber. Thus the secondary air is mixed with the exhaust gases discharged from the combustion chamber, and the mixture of the exhaust gases and the secondary air is burned again (oxidized) in the exhaust gas cleaning section, thereby cleaning up the exhaust gases. Furthermore, the present invention is provided with two valves for checking the back flow of the air while maintaining the air flow from the crank chamber to the exhaust gas discharge section. One valve (first valve) is disposed within the intake passage and opens when the negative pressure is built up in the crank chamber. The other valve (second valve) is disposed within the guide passage and opens when the positive pressure is built up in the crank chamber.