I usually break them in with the mousse can, but I've seen it done both ways.

The break-in itself is important. There are some different thoughts on how to do it though. Basically, you want to run the engine rich enough to ensure enough lubrication to the conrod, bearings, etc. But you need to get the engine up to normal temperature FAST, so you never want to run it so rich that it "four cycles" or spits fuel. Running at too cool of a temp results in the piston/liner wearing for a good fit at that lower temperature. When the enging actually heats up fully, you wind up with a loose fit and a loss of power.

Some guys set the engine rich, then put on a prop with the same pitch, but one inch smaller diameter, then do a short series of very short (10 seconds or so) runs, with a complete engine cooling in between. The small prop allows the engine to turn at the RPM expected later on when leaned out, but the rich setting provides the necessary lubrication. This temperature cycling, in theory, reduces the internal stresses in the metals, and can allow you to break in an engine fairly quickly. At some point, they put on the flying prop, run the engine a bit more, and that's it. But you can also wreck your engine if you do this wrong, and over-rev a new engine. If you want to do this technique, I suggest you read more about it. I think the Jett Engineering website had info on this, and if you search on here or on the RCCA forums at rccombat.com you can find more info.

You can also just do some short runs on the rich side, 2-cycling, but not leaning it way out. A couple of short 10-second or so runs, cooling in between. Followed by longer runs, leaning the engine out a bit at a time, or leaning in then backing out a bit and so on. Usually done with the flying prop.

The key is to avoid running the engine at lower than operating temp for any longer than you have to. No extended rich running, no extended idling, etc.