What is a common cause of spark plug fouling?

Spark plug fouling typically occurs when excess fuel is present during the combustion process, which is precisely what happens with a rich mixture at idle. A rich mixture means there is too much fuel compared to the amount of air in the combustion chamber. This leads to incomplete combustion, resulting in unburned fuel that accumulates on the spark plug. Over time, this can create deposits that hinder the spark plug's ability to fire properly, ultimately affecting engine performance.

In contrast, a lean mixture at idle could lead to different issues, such as engine misfire or overheating, but it does not usually cause fouling. Overheating of the engine may lead to other problems like pre-ignition or detonation but is unrelated to the fouling of spark plugs. Similarly, incorrect ignition timing could result in poor engine performance or spark knock, but it is not a direct cause of spark plug fouling.

Thus, a rich mixture at idle stands out as the primary and common cause of spark plug fouling, emphasizing the importance of maintaining the correct air-fuel mixture for optimal engine performance.