Which factor increases the likelihood of fouled spark plugs in an engine?

The correct answer focuses on rich mixture settings during taxi, which leads to a significant increase in the likelihood of fouled spark plugs. When the fuel-air mixture is rich, it means there is an excess of fuel compared to air. This can result in an incomplete combustion process, where unburned fuel can accumulate on the spark plugs. This buildup can lead to fouling, making it difficult for the spark plugs to generate the necessary spark to ignite the fuel mixture efficiently.

In the context of aviation engines, particularly during taxi, maintaining a rich mixture can be common as operators seek to ensure adequate power and engine response. However, prolonged operation with this rich mixture without moving into higher RPMs can allow fuel to puddle, resulting in fouling of the spark plugs.

While factors like cold weather temperatures can affect engine performance and ignition, and high-density altitudes can impact overall engine efficiency and power, they do not directly lead to the fouling of spark plugs as significantly as rich mixture settings during taxi do. Low humidity in the atmosphere also plays a role in combustion efficiency but is not as directly tied to spark plug fouling as the mixture settings.