What does a measurement of CO in percentage indicate?

A measurement of carbon monoxide (CO) in percentage indicates a rich mixture, which means there is more fuel relative to the amount of air in the combustion process. In a rich mixture, not all the fuel is fully combusted, leading to increased levels of carbon monoxide in the exhaust gases. CO is a byproduct of incomplete combustion, so higher percentages of CO signal that there is insufficient oxygen to fully combust the available fuel. This is typically a concern in engine performance and emissions control, as it can indicate inefficiencies in fuel use and contribute to air pollution.

The other choices are related to combustion quality; a lean mixture, for example, would have less fuel and more air, leading to lower CO emissions. Efficient combustion represents a well-balanced fuel-to-air ratio where combustion occurs thoroughly, minimizing CO production. Excessive oxygen conditions typically occur in lean mixtures, where the presence of too much oxygen indicates that fuel is not being fully utilized. Thus, an increase in CO percentage is indicative of a rich mixture, primarily due to incomplete burning of fuel.