What is a 4-2-4 break?

If you richen up a 2-stroke just right it only fires every other cycle ("goes into four stroke"). The engine bogs down and the exhaust note deepens. Then if it leans up just a tad it starts firing every cycle ("breaks into two stroke").

In a control line stunt plane, you can take advantage of this by adjusting it to four-stroke when it's level or nose-down, but two-stroke when it's nose-up. That way when you are pulling vertical lines you get more power, and when you're going straight down you get less.

As mentioned elsewhere, some motors will four stroke easily (like the old Fox stunt .36), and others won't.

There's also a trick you can do with a tuned pipe that's made a bit long for the engine speed -- it turns out that when you do this right, the engine has a negative response to load, so that it'll speed up when you load it, and slow down when you unload it. This gives the same overall effect without the needle-valve games, plus it gives you an excuse to have a neato-looking carbon-fiber pipe on your plane.

Ahh, I get it. Huh, I would have never thought of doing that. I don't know if you could do that reliably with the small Cox engines, as there is too big of a gap between 4-stroke mode, 2-stroke mode, and max power 2-stroke mode. Also, for a c/l .049 powered plane I'm not sure you could get off of the ground in 4-stroke mode :p. Are the OS 35 engines one that does that well?

I dunno. I forgot to mention -- all my practical experience in that regard is with a Fox .36; everything else I know I got from reading magazines.