WAIT! If this is an ABC engine, ignore EVERYTHING that's been said above!

The very last thing you want to do with an ABC engine is to run it rich. You'll destroy it pretty quickly.

ABC engines have a very tight piston to sleeve fit at the top of the stroke. This is where they get their compression.

"Breaking in" an engine is done to seat the parts so that they fit properly. With a standard engine, rich keeps the parts cooler so that they seat without wearing excessively.

Well in an ABC engine the piston and sleeve expand at different rates, so if you run them rich (they stay cooler) the piston expands but the sleeve doesn't. Therefore the sleeve wears too much, and when you do lean it out and run at operating temperatures a good deal of your compression will be gone. Not a good thing.

To break in an ABC engine you want to run it at operating temperatures (fairly lean) right from the start. But, you only run it for a minute or two, then shut it down and let it cool down. You continue doing this, and after a couple of tanks you increase the run time between cool-downs.
Dennis-