.....When going full throttle, then turning throttle off about 1/4 turn, there is a missing or sputtering sound. Like a popping out the exhaust, possibly right side.........
Your description sounds just like a "too rich" main jet check. IE: at WOT, quickly reduce to 7/8 or 3/4 throttle and notice what happens. If the engine picks up a little power, the main jetting is too lean, but if the jetting is too rich, power will fall off and in an extreme case, the engine can eight stroke or blubber (fire every other cycle). This can sound just like a misfire.
So, how to check this possibility..... How about try it in the morning when the air is cool. If the symptoms are less bad, that suggests an over rich condition. Do it again in the heat of the day and compare your seat of the pants afternoon results with how it ran in the morning. If morning was better and afternoon was worse, you are jetted too rich.
So then, I would confirm the main jet size is correct and if it isn't, put the correct jets in it. And second, I would replace the needle jets because they do wear out rather quickly and richen the mid range enough to reduce power and if they are super worn out, that could be your whole trouble.
Oh, DavidP is right about that B6 plug. It's too hot and you are risking engine damage. Put B8ES's in it at least. If it needs 6's so the plugs don't foul, that's another sign of being too rich.