

Lean burning increases mileage, but produces nitrous oxides.
Rich burning produces carbon monoxide, which eventually converts to CO2 in the atmosphere.
Unit for unit, NOx emissions are 265 times as damaging as CO/CO2.
https://scied.ucar.edu/learning-zone/how-climate-works/some-greenhouse-gases-are-stronger-others
To successfully convert NOx, catalytic converters need a stoichiometric or slightly rich fuel/air mixture to the engine.
Indeed.
Unfortunately, no matter how well you tune your carburetor, just a change in atmospheric temperature is enough to throw off your mixture.
The only feasible way to adjust it fast enough to keep up with atmospheric changes is with some kind of fuel injection, an ECM, and an O2 sensor to provide the feedback needed for closed-loop mixture control.
Fortunately: https://speeduino.com/