What does a rich fuel mixture indicate regarding engine performance?

A rich fuel mixture suggests that too much fuel is being injected into the engine relative to the amount of air available for combustion. In an ideal combustion process, a specific balance between fuel and air is necessary to achieve optimal performance and efficiency. When the fuel mixture is rich, it can lead to incomplete combustion, resulting in poor engine performance, decreased fuel efficiency, and increased emissions. In this situation, the engine may run rough, have difficulty starting, or experience excessive black smoke from the exhaust, indicating that there is more fuel than necessary for proper combustion.

The other options relate to different potential issues within an engine's operation. For example, insufficient fuel injection would lead to a lean mixture, which causes different performance issues. A clogged air filter would restrict airflow, also impacting the mixture but in a different manner, and would not directly indicate an excess of fuel. Lastly, needing to replace spark plugs could affect combustion but doesn't inherently relate to the fuel mixture itself. Thus, identifying a rich fuel mixture is critical for diagnosing the engine's performance and making necessary adjustments.