Q: Since I got the Yuasa, my bike starts right up. I don't care about the popping (I kinda like it). I believe the plugs look OK. Thus, is there any reason to mess with the fuel mixture screws on the carbs?
A: I'm going to say No...if it ain't broke...don't fix it...( it MAY let your bike run smoother at idle however..) Knifemaker
Those copper rings are the crush gaskets. If they're completely flat, they've been "crushed" and should probably be replaced. Someplace I heard that they are pretty much a single-use item, and whenever you loosen or remove the header pipe, they should be replaced. No promises, but I'm guessing there's a good possibility that replacing them with the HD gaskets or the ones from JUDGE would help your popping problem. I got mine from JUDGE ($6.00 for a pair), and they are just hollow metal rings with a slit that will "crush" when you tighten the acorn nuts on the header pipes.
Sorry...forgot to give you the part #..is 17048-98 at any harley parts department.
You say that your gaskets at the cyclinder heads are copper ring flat gaskets. I suspect they might have been "mashed" flat. The cylinder head gaskets should have some "expansion" capability. The gap between head pipe and cylinder head expands and contracts as engine heats and cools. Try new crush gaskets to eliminate popping