Hi Mark,
I had the same problem as you with my TF P-51 with a DA 50.
I think these type airplanes have good enough air flow past the cylinder head.
The problem I think is the tight cowl up by the carburetor.
I didn't like doing it .....but I took my Drimel tool and made a small narrow slit on top of the cowl to force air down to the carb.
I also have between the engine supports a sheet metal air dam zip tied to keep the muffler heat down.
I went back to DA factory needle settings also that helped.
It work for me.