Which engine condition is most likely to produce fouled spark plugs?

Fouled spark plugs are most often associated with low power settings combined with a rich fuel mixture. When the engine operates at low power, especially in a rich mixture scenario, there is insufficient heat generated to completely burn the excess fuel. This unburned fuel can lead to carbon deposits accumulating on the spark plugs. Over time, these deposits build up, causing the spark plugs to misfire or fail to ignite the air-fuel mixture properly.

In contrast, operating at high power with a rich mixture might also lead to fouling; however, the higher temperatures produced in such conditions typically help burn off unwanted deposits more effectively. Meanwhile, low power conditions with a lean mixture usually lead to a hotter running condition, which further reduces the chances of fouling since the spark plugs can reach the necessary temperatures to minimize deposits. Therefore, the combination of low power and rich mixture creates an environment conducive to producing fouled spark plugs.