There are several things that cause the pop sound....First is a leaking exhaust gasket. Second is a lean condition which can be adjusted by the fuel mixture screws you are asking about. They should be turned in until they bottom out (gently so as not to damage the seat) then turned back out anywhere from 2 to 3 full turns. There is one on each carb as shown in the pictures. The third is the emission control stuff on the bike. You can block these with the marble trick or coasters. There is info for this in Vulcan Verses as well as multiple threads here. Hope this helps.
And KM, I believe your calculations are off a bit. I got a HP increase of .003% and my bike runs .05% cooler. Perhaps the extra .00000001% of a turn I gave my screws is the difference. You need to readjust yours....lmao.