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<blockquote data-quote="SRAD97750" data-source="post: 209028" data-attributes="member: 425">Many two strokes have an exhaust valve which blocks some portion of gas flow through the exhaust port.It ultimately changes port timing/duration so the engine can make power at more than just a narrow RPM range.Typically at lower RPM, the valve makes the exhaust port shorter/smaller. As engine RPM increases, and exhaust duration should increase, a speed governor (typically spring governed) opens the valve.Exhaust valves come in many shapes and sizes and I am just amazed at the ingenuity that goes into the hundreds of exhaust valve designs. Some are like saloon doors, some are additional ports, some are like a garage door, and I've seen rotating pillars with variable cutouts on each side. Amazing stuff.The exhaust valve system typically requires a good cleaning on every inspection because it operates directly in the exhaust gases which are rich in unburnt oils. Usually you set them from the factory specs and never think about them again, other than routine cleaning. -BIG DAN<img src="https://www.dirtbikeaddicts.com/static/images/smilies/thumb.gif"&nbsp; class="smilie" loading="lazy" alt=":thumb:" title="Thumb&nbsp; &nbsp; :thumb:" data-shortname=":thumb:" /></blockquote>

[QUOTE="SRAD97750, post: 209028, member: 425"] Many two strokes have an exhaust valve which blocks some portion of gas flow through the exhaust port. It ultimately changes port timing/duration so the engine can make power at more than just a narrow RPM range. Typically at lower RPM, the valve makes the exhaust port shorter/smaller. As engine RPM increases, and exhaust duration should increase, a speed governor (typically spring governed) opens the valve. Exhaust valves come in many shapes and sizes and I am just amazed at the ingenuity that goes into the hundreds of exhaust valve designs. Some are like saloon doors, some are additional ports, some are like a garage door, and I've seen rotating pillars with variable cutouts on each side. Amazing stuff. The exhaust valve system typically requires a good cleaning on every inspection because it operates directly in the exhaust gases which are rich in unburnt oils. Usually you set them from the factory specs and never think about them again, other than routine cleaning. -BIG DAN:thumb: [/QUOTE]

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