Copied directly from Ron Lund's FAQ located at http//:http://www.ronlund.com
What is the best way to break in my new engine?
I would imagine you all have your favorite ways, and some of you don't bother. Those of you who don't bother are missing the boat. I was in that group for a long time. I figured it didn't make any difference anyway. WRONG. An engine that is broken in properly vs. one that is slam dunked from day one will have up to 30% more power over it's lifetime. That's an estimate, but it's from a good source. Curtis Youngblood, well known for having really powerful engines, shared his "quick" break-in procedure with me. I've tried it and it works really well, is quick and doesn't keep you from flying immediately. I've added a couple things that I do when I want my engine to work really well. I'm assuming you can tune your engine and you have the throttle/pitch curves set up properly.
There are basically two types of engines today. Ringed and ABC. For the most part the break-in is the same for both, however on an ABC engine, substitute "a rich, but solid two cycle" for "rich". Running a ringed engine really rich doesn't hurt anything, but in an ABC engine, you aren't doing it any favors by running it slobbering rich. The only thing that wears is the connecting rod. An ABC engine must get up to temperature to run right. Unless it gets to temperature, it won't ever break-in.
I'm going to try to keep this really simple, but bear with me. Needless to say, the first step is to install the engine. Before you do, make sure you've removed the back plate and inspected the inside for foreign material. It's not a bad idea to put a few drops of fuel in it and spin it around a few times. Not too much. Make sure you've dial indicated your fan/clutch to make sure there isn't excess run out. If you don't know what I'm talking about, buy Ray's Manual and read it. It's hard enough on an engine during break-in. Don't add a foaming fuel tank to your work.
Set your needles to the recommended setting. Pinch off the fuel line to the carb and fill the tank full. Remove the glo plug and spin the engine over very briefly to make sure there's no oil in the combustion chamber. You don't want to break your rod before you hear the engine run. Install the glo plug, open the fuel line and light the plug. Start the engine. If your engine doesn't start immediately, don't spin it until you wear down the battery on your starter. Find out what's wrong and fix it. Either the glo plug isn't lit or it's not getting fuel, simple as that.
Once you have the engine running and tuned for a rich mixture, especially the idle. Hover it for about 30 seconds and set it down. Let it idle for about 20 seconds and cool off. Repeat this, adding 30 seconds to each hover until the first tank is gone. Don't run the tank dry. The idea here is to heat it up and cool it down, which is a very important part of break-in. If at any time the engine starts to go lean, shut it down immediately and find out why.
Refuel and now follow these three rules for the next 15 tanks of fuel. It's ok to fly around, even do small loops and rolls. No high power maneuvers...period. Just follow the rules.
1. Don't overload the engine. No full throttle...period.
2. Don't over speed the engine. If you can't descend without over speeding, auto it in, or don't go high to begin with.
3. Don't run it lean
That's it. You'll notice the engine making more power toward about the tenth tank. Don't be tempted to overload it until you've finished the break-in.
Keep the fuel clean. Keep dirt out of the engine and you'll have a long lasting powerful engine.