We've all been there.
Running great. Then - all of a sudden - not so great.
Ah, dirty idle jets in the Webers! Pull them, clean them, reinsert, and - no difference!
And you didn't see any dirt in the jet, anyway, right? And you blew out the 'hole' the jet goes into with carb cleaner, too.
This video explains why you probably didn't find any dirt and what's really going on.
Also, without saying so directly, it shows why tweaking a mixture screw might sometimes have no apparent effect on how the engine is running.
Best video I've come across that shows how the whole idle circuit works. There's a lot more to it than just the jet.
Make some popcorn and watch.