What happens when the air fuel ratio is rich?

When the air-fuel ratio is rich, it means there is more fuel than air in the combustion mixture. This can lead to incomplete combustion, resulting in increased emissions of unburned hydrocarbons and carbon monoxide, as well as a decrease in engine efficiency. Additionally, a rich mixture can cause fouling of spark plugs and increased carbon buildup in the engine, potentially leading to engine performance issues and reduced fuel economy.