I gather Webers tend to give running trouble when they're too lean, whereas too rich they run mostly okay, just dirty with poor fuel mileage.

Evidence of a rich condition (like sooty plugs) can be misleading, as that could also be due to a lean misfire. If you had access to an exhaust analyzer, this would show high HC and low CO, whereas rich would show both high.

BTW, the idle jets aren't just for idling; they also serve most low-RPM driving, then the mains start to come into play at mid-high RPMs.