I didn't find the difference in sound to be as noticeable as I expected when breaking in my first (46AX) engine. Now it's as clear as night and day.

When it's in four stroke, it will have a certain pitch. As you turn the needle slowly to the right, you'll get to a point where the pitch seems to jump between two frequencies (i.e. four and two stroke). When you get to this point, turn back to the left 1/8 turn or so, and you're in 'four stroke' operation. Conversely, two stroke is the higher of the two pitches, and it is in two stroke operation just when it stops jumping from low to high, and just stays in the higher pitch.

Just want to reinforce the point of having the plane secured, which is especially important if you are alone. I used a screw-in type dog run and a dog leash looped over the tail of my plane. With my nice planes, I now use two garden stakes with foam pipe insulation around them and brace the wings.

[link=http://www.hooked-on-rc-airplanes.com/nitro-engine-tuning-tips.html]Here's a link on basic tuning.[/link] There are better ones out there I'm sure, but this is a start.