Hey, I'm glad your Thunderbird is running, even if difficult to start. I had and still have similar difficulties after disassembling and cleaning the Mikuni carbs on my 2000 T'bird. I needed to do that after the bike sat for 18 months with a tank and bowls full of gas with 10% ethanol. Found nasty green crystalline substance plugging jets and passageways. I didn't find any loose brass parts in the bowls fortunately. Yours may be the brass plugs that the factory uses to close air and fuel passages drilled in the carb body. That might explain difficult starting due to an internal fuel or air leak. In any case, keep riding.