best air fuel ratio

[John Wallace]

I believe the ratio is for mass not volume?

For 12.5 pounds of air, you need 1 pound of gasoline?
Now at different barometric pressures, there is less pounds of air per 1 cubic foot of space.
(like in cfm of carb)
Gasoline more or less is still the same pounds per cubic foot of space.

If there is water in the air, that means less O2 in that same cubic foot of space.
(grains of water)

At Denver there is a lot less O2 in that same cubic foot of space.

Need to think in mass not volume?

Also the stoichiometric value is for the optimum reaction of O2 combining with gasoline, CO2 is a byproduct of the reaction.
(along with H2O and NOx, heat, other stuff )

If the engine is mis-firing it could have a ton of CO2/CO emission but the AFR is spot on?

[treyrags]

All true. Also in general, that less dense air will draw less fuel through the boosters of a carb to compensate.

[jmarkaudio]

On an Innovate wideband a misfire will spike lean.

[headman]

If an engine is misfiring, it will have a ton of unburned HC.
When tuned for best power the HC ppm will be lower.
Course, the timing being too high will show increased HC ppm also.