I think that's a ringed engine? In that case it's no where near run in yet and 15% nitro might be a bit much for it anyway.

But it's definitely loading up at idle. Inverted engines are much more critical to idle mixture than what you can get away with normally. Use the pinch test to set the idle and aim for only a very slight rise in revs. You could try an idle bar plug too. Flypaper had good advice...sometimes the obvious gets overlooked.