[schuwolf]

Damo's link contains good info.. Make sure you 'heat cycle' the engine during run in, use an old sock or tin foil to keep the head covered and get the temps up, no more than 230F. Allow it to cool completely before the next firing. A temperature gun is invaluable here. After a few tanks you can start to lean out the HSN by 1/8th of a turn a time. Keep a check on temps and aim to settle down around 230-270F and NO more than 275F. Once run in go for full throttle runs on the sort of surface you will mostly drive it on and check temps and maintain a nice plume of smoke. Buy a decent brand of fuel like Byron's 25% and keep to it. Expect a few glo plug failures during running in, probably need a medium to hot plug with 25%. What engine are you breaking in?