The carb setting is too rich, probably on both needles.
First lean the idle needle, until the engine just about fails to cleanly accept throttle horsing. (quick forward stick). That would be the right needle setting.
Then fly, and check the plug again. The idle leaning will also have leaned out full power a bit. That is why you first have to get the L-needle right.
Full throttle flight should have the engine running cleanly, even on speed downlines. Uplines should be without sagging.
Try to chop the engine while at full bore, and glide in and let the engine cool down to check the plug.
Never put a wrench to a plug on a hot engine. That will damage the threads.

edit:
PS, I have a servo delay on my throttle servo. For the abovementioned method I set the throttle speed to 100%. I then move it back to 70% speed, and my warm engine will then accept horsing the stick without a hickup.