What is the most likely cause of an engine missing under load?

An engine missing under load is most commonly attributed to a faulty ignition system. When the ignition system is not functioning properly, it can lead to an inadequate spark or a misfire, particularly under the added stress of a load. This failure to maintain the correct timing or intensity of the ignition can disrupt the combustion process in one or more cylinders, resulting in a noticeable miss.

While other factors, such as a clogged fuel filter, lack of air intake, or excessive engine load, may contribute to engine performance issues, their primary effect is typically more related to overall engine performance or fuel delivery rather than specifically causing a misfire during load conditions. For example, a clogged fuel filter would lead to inadequate fuel reaching the engine rather than a specific ignition failure, and a lack of air intake would generally result in a sluggish performance rather than a distinct engine miss. Excessive engine load itself can exacerbate existing issues but does not inherently cause an ignition-related misfire.

Thus, a faulty ignition system is the most direct and probable cause when diagnosing an engine miss under load conditions.