This engine is made for 15% nitro so lowering the nitro won't make much differance and since it is made for higher nitro would actually run a bit hotter (I doubt it would be enough to notice) not cooler. The only time reducing nitro will make it run cooler is when it is detonating. But then a cooler plug would have the same effect. The issue is either air cooling, which I doubt is the problem because of the huge opening below the engine and head fins in the breeze. Or running lean, most likely after it has run a while and mostly when climbing.