Actually it sounds like you are pretty close. The divider in the port definitely means it is a 25cc. I noticed during the one flight shot that it four stroked during a dive. That can happen with two stroke gas engines, especially if your prop load is a bit light in general, or if your pitch is a bit high on the prop. The engine is all set up and tuned with a static prop load. Then in a dive, especially if the pitch is a bit high on the prop, the engine "unloads", (is not working as hard) and the engine has too much fuel for the load and therefore four strokes.

Keep leaning the low needle in small increments until you lose good throttle transition. Then go back 1/16 to 1/8" turns richer.

But really, the flight sounded pretty good. Just a few burbles here and there, no big deal... Walbro says in their tech literature that an engine might need to be richer than optiminum at idle/midrange for good throttle transition. This will often make them ocassionally four stroke in flight. If it really bothers you, then keep leaning it on the low end, and learn to advance the throttle slowly. Remember to reset the high end each time you change the low end, as one affects the other.

AV8TOR