1.  Field of the Invention
The present invention relates to a two-cycle engine, and particularly to a rotary valve type two-cycle engine. 
2.  Description of Prior Art
In a two-cycle engine to be equipped such as in a motorcycle, in order to raise the engine output, the intake resistance is needed to be possibly reduced, to thereby increase the efficiency of intake charge into a cylinder.  For the reduction of intake resistance, it is generally effective to shorten the length of an intake path.  In this respect, a best arrangement to shorten the intake path length includes a rotary valve directly provided at one end of a crankshaft, and a valve opening formed in a crankcase and connected to an intake port, while a carburetor is disposed just behind the intake port. 
In this arrangement, however, the crankshaft is directed transversely of a vehicle body, so that the transverse width of engine is increased, thus restricting a banking angle in the case of a motorcycle which is let to bank when cornering. 
For such reason, there has been proposed what is called a rotary intake valve backing type two-cycle engine, in which an intake port is arranged in the upper part of a crankcase perpendicularly to a crankshaft, thereby reducing the transverse width of the engine, and a rotary valve interlocked with the crankshaft is disposed in the intake port. 
However, the rotary intake valve backing type engine necessarily requires a longer intake port than a side mount type engine which has, as described before, an intake port arranged in the axial direction of a crankshaft.  In this respect, in a rotary intake valve backing type arrangement in which an intake port is disposed in the boundary region between a cylinder and a crankcase to thereby shorten the intake port length, the sectional area at that end opening of the intake port which is located on the cylinder side becomes restricted with a cylinder sleeve fronting the end opening.  Moreover, in a modified arrangement in which the length of a cylinder skirt is cut short to secure a necessary sectional area at an end opening of an intake port, the cylinder skirt fails to effectively hold to support a piston which may thus be caused to unsteadily swayed, when approaching its bottom dead center. 
Further, different from the case where a rotary valve is installed at either transverse side of a crankcase, thus permitting a carburetor to be directly mounted on a valve cover, in an arrangement having a rotary valve mounted from above on a crankcase as above-mentioned, the associated space layout generally requires an intake tube for connecting a carburetor to an intake hole formed through a valve corner, thus needing a clamp band or the like.  In such arrangement, therefore, an annular extension for fitting thereon a clamp band is projectingly formed around an intake hole of a valve cover, whereas the necessary securement of space such as for the clamp band is to unsuccessfully shorten the intake tube length, thus failing to reduce the intake resistance, besides that an installation work of the intake tube in a confined space is quite troublesome. 
On the other hand, in the field of internal combustion engine, to prevent pressure variations in an engine casing such as due to the variation of temperature, there is generally employed a breathing system including a breather chamber for effecting the breathing of air as separated from fuel-air mixture, the breather chamber consisting of a complicated route or the like formed in the engine casing.  In the art of a two-cycle engine adapted to be mounted such as on a motorcycle and thus required to be possibly light-weighted and small of size, however, a sufficient space is difficult to secure for the capacity of such a breather chamber, whereas the securement of breathing capacity necessarily gives rise to an enlargement of the engine casing, resulting in a larger size, thus unsuccessfully achieving a light-weighted design.  Particularly, in rotary valve type two-cycle engines, besides the before-described disadvantages as, there occur shortcomings such as various restrictions in the layout of a breathing system of which breather chamber is required to be arranged so as not to interfere with a rotary valve, thus frequently resulting in the difficulty of disposing the breathing system in a desired position. 
The present invention has been achieved to effectively overcome such conventional problems in the art of a rotary valve type two-cycle engine.