Two-stroke power valve system

As the piston passes the midpoint of the downstroke, the exhaust port to the side of the cylinder is uncovered and initiates the flow of burned fuel into the expansion chamber or muffler.

It is the same simplicity in design, however, that causes a two-stroke engine to be less fuel-efficient and produce high specific levels of undesirable exhaust gas emissions.

This can allow a significant amount of fresh fuel to run straight through the engine without being burned in the process of power production.

These valves act to vary the height (and width) of the exhaust port thereby broadening power delivery over a wider rev range.

Exhaust ports with fixed dimensions only produce usable power in a narrow rev range, which also affects fuel consumption and emissions.

Power valve actuation can be by mechanical (RPM dependent) or electric (servo motor) means increasingly with electronic control.

Electronic control offers a greater degree of accuracy as well as being able to vary the opening of the valve and be tuned to conditions.

It was also added to the DT(125lc 2/3) range after 1984 (but was locked closed to comply with UK learner regulations until the (R) in 1988-04 which had a fully functional YPVS valve) the DT125R has a better design of engine, although not much altered in speed, just more reliable than its predecessor.

The YZ series of motocross bikes has a mechanical power valve which is activated at RPM speed.

The 1977 OW35K was the first race bike to incorporate the power valve system and it won the Finnish GP in 1977.

The Kadenacy effect was harnessed and controlled to a point that gave Yamaha great advantage over all the other manufacturers throughout the late '70s and into the mid '80s.

At high RPM the ATAC valve is closed and the exhaust simply exits into the expansion chamber.

A larger expansion chamber allows for more power at lower RPMs because of the extra time needed for the impulse to "bounce" back for the supercharger effect.

A small computer monitors engine RPM and adjusts a two-blade exhaust valve with an electric servo.