OK 15% means 15% nitro content it is less powerful than 30%. normally the higher the nitro the hotter then motor runs. I have run 15% in my hawk with an os .32 and now I am running 30% because I want more power out of it as I am doing some aerobatic moves. It is true that with higher nitro you can run richer and still have tons of power to fly, where as with lower nitro as in 15% you can't run too rich or you will loose power and won't be able to lift. It sounds as though you are running too lean, and maybe try changing the glow plug, I neede to after switching nitro content. Get a temp gun and check the temp of your motor when it loose power I run mine around 198 to 220 degrees at the base of the engine oppisite the muffler. I have NO problems with mine dying out and I can fly all day 1 tank after another and I fly hard. Hope this helps you. Alan