You may have an air leak somewhere that is giving you fits on adjusting your carb. Is it leaking fuel from around the intake anywhere? There should be an o-ring where the carb seats into the the intake tube, and (if I remember correctly) another o ring where the intake manifold goes into the cylinder head. make sure those are there and tight. Sometimes with an air leak, one has to adjust the carb real rich to get it to run right, and then, two clicks leaner, all of a sudden it's too lean and dying.
15 percent is exactly what I run in mine. I think it has a 12X6 prop as well, that sounds right.
If I remember right, we clocked my P51 at around 90 mph or so with that engine, so it is plenty capable.