The exhaust valve(s) you speak of.... On the Enya, (and others), it was done to keep the glow plug warm at low speeds and idle. It didn't change the exhaust port timing; just provided some back pressure to keep things warm at low throttle settings.

On the newer two stroke engines with an exhaust valve, the valve actually changes the exhaust port timing, allowing you to have a more radical timing at high rpms, and a lower, more tractable timing at low rpms. Rotax also uses this technology, among others.

AV8TOR