What happens when a mixture is too rich?

A mixture that is too rich refers to a condition where there is an excess amount of fuel in relation to the amount of air in the combustion process. This imbalance leads to incomplete combustion, which can result in several unwanted effects.

When the mixture is too rich, one significant consequence is the formation of deposits in the combustion chamber. These deposits arise because the unburned fuel can accumulate and carbonize, creating sludge that adheres to engine components. Over time, these deposits can affect engine performance, leading to issues such as rough operation or decreased efficiency.

In contrast, the condition of increased engine power, optimal fuel efficiency, or reduced engine temperature typically occurs in situations where the fuel-air mixture is balanced or slightly lean, not when it is too rich. Thus, the formation of combustion chamber deposits is a direct consequence of an overly rich mixture, underscoring why this option stands out as the correct answer.