What happens when an engine runs with a "rich mixture" of fuel?

When an engine runs with a "rich mixture" of fuel, it means that there is an excessive amount of fuel relative to the amount of air being mixed for combustion. This condition can lead to wasted gasoline because not all of the fuel is burned efficiently in the combustion process. As a result, some of the unburned fuel can escape through the exhaust system, contributing to increased hydrocarbon emissions and pollution.

Moreover, while a rich mixture can sometimes temporarily enhance engine performance under certain conditions, the overall efficiency of fuel usage declines, resulting in less fuel economy and potential environmental impacts. This explains why running an engine with a rich fuel mixture generally leads to wasted gas and increased pollution as the system struggles to utilize the excess fuel effectively.