On my recent engine build I noticed something interesting. You know those big washers with a thin black rubber washer that go on top of the CB air cleaner tops? When new, little tiny pieces of rubber come off and fall through the stud holes RIGHT ON TOP of the carb every time they are removed or installed. Where do you think they end up? Right in the idle circuit air intake. After a while, the rubber gets worn enough that rubber stops chunking off.
I noticed this too, so I used an exacto blade and removed the rubber that touches the threads. I also cleaned the threads while holding the shop vac hose to catch the crud. I guess one could also use a drill bit or a small round file to remove the excess rubber.