Too much nitro, unless you add a lot of head gaskets. I run 7% nitro, 11% castor, 11% synthetic. 5.5 x 2.5 APC prop. Unlike Cox engines, these will overheat and run badly if at all over propped, over nitroed or over compressed. Let the engine sing a 19k note on the ground minimum.
If you want the extra power that Nitro yields, you must add extra gaskets.