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Old 02-20-2012, 06:29 AM   #17
Gnarly Adventurer
Joined: Feb 2010
Location: Lower Slobbovia
Oddometer: 306
I have a donor '74 175 and a running '73 175. What do you want to know?
Here's a brief overview:
-Kawasakis were the main user of the rotary valve engine of the Japanese bikes but CanAm's used them with great success.
-My Clymer lists 80-350cc models from '66 t0 '94

-The rotary valve opens to allow the fuel mix into the crank cavity and then closes until the down stroke is complete. Thios keeps the charge in the crank instead of spitting back through the carb.
For early two strokes this was a great way to broaden the powerband. The exhaust pipes didn't use the exhaust pulses to scavenge the spent exhaust very well. If you look at a modern two stroke pipe it is fat and thin in different areas to make the best use of this exhaust pulsing to suck the spent gases out of the cylinder.

Yamaha used reed valves for a similar effect. The flexible reed opens when the engine sucks in the fuel mixture and closes when the exhaust back pressure pushes back. It's located in between the carb and cylinder.
The reed valve works best for increasing the low end power and becomes useless at higher rpms as it stays open.
The main disadvantages of the rotary valve are complexity, and increased case width.
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