Mchwind Chimes,
The best way to break in any engine is to follow the instructions in the manual that comes with the engine. If you follow these instructions you can't go wrong. If for some reason you don't have the instructions that came with the manual you can find an online copy here, [link=http://www.osengines.com/manuals/index.html]OS Engines Manuals Online[/link]
Here are the instructions copied from the manual for the OS 40 LA
RUNNING-IN ( "Breaking-in" )
All internal-combustion engines benefit, to some degree, from extra care when they are run for the first few times - known as running-in or breaking-in. This is because the working parts of a new engine take a little time to settle down after being subjected to high temperatures and stresses. However, because O.S. engines are made with the aid of the finest modern precision machinery and from the best and most suitable materials, only a very short and simple running-in procedure is required and can be carried out with the engine installed in the model. The process is as follows:
1) Start the engine and, with the throttle fully open, open the needle-valve an extra half turn (180°) from the optimum setting. This will produce a rich mixture that will result in cooler running. Allow the engine to run out a full tank on the ground. (Avoid dusty surroundings.)
2) Now fly the model with the needle-valve re-set 20- 30 degrees open from the optimum setting ( i.e. 40-60° from the highest rpm setting ).
3) Close the needle-valve very slightly on successive flights so that the engine is running on its optimum needle setting at the fifth or sixth flight.
I hope this helps
Ken