Power Valve Tuning
What is a power valve?
The power valve is a movable flap located on the cylinder exhaust port of a two-stroke engine – where burnt gasses are flushed out of the cylinder. At low RPM, the flap is held partially closed to make the exhaust port smaller. This helps the engine generate torque at low speeds. As revs build, the flap gradually opens, expanding the size of the exhaust outlet for greater top-end power. The ability to vary the size of the exhaust port gives your two-stroke engine broad, usable power.
Why adjust it?
When in the RPM range the the power valve opens, and how fast it opens, affects the power delivery characteristics of the engine. The ideal power delivery isn't necessarily the same for all riders or riding conditions, so many riders find it beneficial to alter power delivery characteristics to suit the available traction and their personal preferences.
How do I adjust it?
In the KTM 250/300 engine design, the power valve is held closed by a pair of springs, and exhaust gasses force it open as they increase in velocity and push harder against the flap.
There are two ways to adjust the KTM 250/300 power valve:
1) Change the main spring preload. This adjustment determines when the exhaust flap starts to open. This is what the PowerDial helps you adjust.
2) Change the auxillary spring. Your KTM came with green, yellow, and red auxiliary springs. Change the spring to make your exhaust flap snap open quickly (red spring) or open gradually (green spring).
This chart shows the effect of auxiliary spring choice on power valve opening behavior in the KTM 250/300 engine.
Dynamometer runs depicting the effect of power valve preload settings. Each line represents 1/2 turn change on the adjuster, with purple "1T" in the fully counter-clockwise position. Test conducted with yellow auxially spring.