What happens if the air-fuel mixture is too rich?

When the air-fuel mixture is too rich, meaning there is an excess of fuel in relation to the amount of air, several detrimental effects can occur in an engine's performance and longevity. A rich mixture can lead to incomplete combustion, which results in unburned fuel exiting the exhaust system. This not only wastes fuel but can also cause the engine to run inefficiently, leading to poor fuel economy.

Moreover, a consistently rich mixture can lead to carbon build-up on engine components, such as spark plugs and valves, potentially causing misfires and reducing overall engine performance. In severe cases, the excess fuel can wash away the lubricating oil from cylinder walls, which can lead to increased wear and ultimately engine damage.

Thus, running an engine with an overly rich air-fuel mixture affects both operational efficiency and could lead to significant mechanical issues over time, making this answer the most accurate in describing the negative consequences of a rich mixture.