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Kawasaki rotary inlet valve was a component of the fuel and air intake system and a major part of the success of early Kawasaki single cylinder and twin cylinder transverse two-stroke motorcycle engines. Kawasaki Motorcycle Corporation was using the rotary valve as early as 1962.

Kawasaki design
The Kawasaki single cylinder and twin cylinder engine rotary discs utilized either a pressed fiber or stainless steel symmetrical disc mounted on the crankshaft with a raised lug. The disc has a corresponding notch for ease and accuracy during replacement and with the lug, insures an absolute correct placing of the disc on the shaft without error. The disc is held in place on the shaft by the rotary disc cover and 5 screws to secure it to the crankcase. The disc cover also has a hollow cylindrical duct protruding outward and away from the engine's center, near the top of the cover which in-turn supports the carburetor via a male/female clasping.

The disc itself, whether fiber or stainless steel, has a port or pathway created by a section of disc which is void of disc material. This is the angular port width with an opening angle and closing angle.


 * Opening angle is the crankshaft angle before top dead center at which the disc's port starts to open and allows fuel and air mixture to enter.
 * Closing angle is the crankshaft angle after bottom dead center at which the disc's port becomes closes to the cylinder.

The rotary disc is capable of being modified from a single uncut disc or existing cut disc. With an existing disc, the closing and opening angle degrees can be increased by removing additional disc material through grinding. This would allow the fuel and air mixture to enter from the carburetor through the disc cover port and into the cylinder sooner and before the piston is at bottom dead center (BDC). By increasing the closing angle, the cylinder receives more fuel and air into the cylinder as the piston is returning on the upward stroke.

Comparison
The advantages of the rotary disc over competitors Honda, Yamaha, and Suzuki with the same engine displacement and number of cylinders was the increase in horsepower and overall performance by allowing more fuel and air in. For example, the Kawasaki F7 175cc produced 21 hp while the Honda SL175 (twin cylinder four-stroke) produced 19 hp, the Suzuki T125 T (piston port) produced 15 hp, and the Yamaha DT175 (piston port) with 14 hp. For twin cylinder motorcycles, the Kawasaki A7 Avenger 350cc produced 42 hp while the Honda CB350 produced 32 hp, the Suzuki T 350 produced 36 hp, and Yamaha R5350 with 36 hp.


 * List of rotary inlet valve Kawasaki motorcycles

Operation
With a rotary disc engine, the intake pathway is opened and closed by the rotating crankshaft which covers and uncovers an opening in the end of the between the rotary disc cover and crankcase, allowing the fuel and air to enter or cease entering during one portion of the cycle.

The advantage of a rotary valve is that it enables the two-stroke engine's intake timing to be asymmetrical which is not possible with two-stroke piston port type engines. The two-stroke piston port type engine's intake timing opens and closes before and after top dead center at the same crank angle making it symmetrical whereas the rotary valve allows the opening to begin earlier and close earlier.

Rotary valve engines can be tailored to deliver power over a wider speed range or higher power over a narrower speed range than either piston port or reed valve engine. Where a portion of the rotary-valve is a portion of the crankcase itself it is particularly important that no wear is allowed to take place.