The RFS stands for Ringed Four Stroke. Magnum is pretty specific about their break-in instructions for this motor, they recommend a series of short (two to three minutes) runs at full throttle followed by cool down periods until you have half an hour of run-in on the motor before you fly it. You very, very gradually start to lean the high end needle toward just-rich-of-peak throttle during these short runs.

anotherhobby will have more long term success by continuing to follow Magnum's break-in instructions.