too add what i know.....from what ive been reading on super high mileage carburators, they have no emissions since they literally burn almost all of the gas because it gets vaporized, and no unburned fuel gets put through the exhaust. what you end up with is literally no emissions, and air that is cleaner than some that you breathe. so it would go to say that if you burn rich, then your putting more unburned gas through, and your creating more emissions. if this is true, then the manufacturers would want to create less emissions, hence they would run their engines a bit leaner...... does this make any sense?