Gare
I have an old Magnum 40GP engine I broke in 3 years ago. I looked up the directions for break-in on the manual that I have and it says to use 10% nitro with at least 18% oil synthetic/castor mix. First tank to be run very rich and start with 2 1/2 turns out on the needle. If the engine starts to die when you remove the glow starter turn the needle value it until it just keeps running. You want the engine to run as rich as it can on the first tanks. The next 2 tanks learn it out about a 1/4 turn more after each tank. Now your are almost ready to fly. On the fourth tank learn it out until you reach optimum RPM than back it out 3 clicks. Do the pinch test when it is at high rpm. The engine should change rpm that return to running speed. Remember to test the engine by picking it up and pointing it to to the sky at a 45% angle with it running at high RPM. If it does not die than you are set to fly. If it dies right away it is to lean, if it sputters it is to rich.
I hope this helps. Good look with your Magnum 40. My 40GP has been a very good engine for the last 3 years.
