I replaced the carb-to-manifold bolts with longer ones so I could apply more torque without fear of stripping the threads and now have idle jets that actually respond as expected.

However, there's now a loud whistle coming from the air horn! This is the first time I've experienced that in 50-odd years of casually working on cars, motorcycles, or small gas engines.

This thing is driving me crazy - I feel as if I'm playing whack-a-mole.