cutaway,

Generally, using a smallish prop, high castor content (~28%) using short runs on a bench for heat cycling are a good way to go on iron/steel engines.

Start rich, run for one or two minutes, shut it down by pulling fuel line or use small tank...don't flood it to stop it. Let it cool to air temperature, then do it again. Every other run, turn the needle in a few clicks. If during any run it starts to sag, shut it down, let it cool, and start it again a few clicks richer.

Once it runs near peak without sagging, run it in a plane for final break-in. Flying some lazy eights will run it between rich and lean for the final heat cycles.

Is this the only way? Of course not. But it is a way to break it in without risk of running over-heating.

Good luck.

George