when i run in an enagine, i use the fuel i will use when im racing it. for starters, i preheat the engine to relieve the stress on the conrod, then if you remove the glow plug and prime the fuel to the carb by the starter box, so its well lubed. then make sure there is still heat in the engine or i preheat agian with the glow plug installed. i then start the engine and let it idle for a tank. let it cool down to room temp thoroughly, then repeat the process again for another tank. after 2 tanks, a preheat of the engine is all i do till its run in at the track for another 6 tanks. replace the glow plug and then race tune leaning little by little for another 2 tanks. hope this helps, as im yet to destroy an engine by doing it this way. they loose compression first and get sluggish.
the trick is to keep lots of fuel going through it at all times, and dont run too hot or cold, both ends of the scale are not good. bout 180f i run them in at. cheers.