I have recently acquired a MVVS 15cc engine and on its first run it started without too much fuss but on the second tank the bottom end needle vibrated out and I lost the spring. Then I assembled the engine I noticed that the spring was very weak and that the bottom end needle turned feely. I have replaced it with a, Parker ball point pen spring and it seems to be ok now. I would like to know if I received the right type of spring with the engine and if it is normal for the bottom end needle to be so lose that it can vibrate out? I understand that one should not use more than 5% nitro with this engine but I suppose this applies to see level. I fly at a 1350 meters above see level and I were wondering if at this altitude it would be ok to use 10% nitro ?