During a ground check, if an engine is rough-running with normal magneto drop and higher than normal manifold pressure, what might be the cause?

When an engine is rough-running with normal magneto drop and higher than normal manifold pressure, the indication of a dead cylinder can be attributed to a few key factors. A dead cylinder occurs when one or more cylinders are not firing properly, which can lead to a rough running condition. This situation manifests as uneven power delivery and can often be felt in the operation of the engine during a ground check.

In this scenario, the normal magneto drop suggests that the ignition system is functioning correctly for the remaining cylinders. However, the expectation of higher than normal manifold pressure alongside the rough running indicates that despite some cylinders firing, one or more are not contributing effectively to the overall engine performance. This would result in a discrepancy between the expected power output and the actual output, leading to the observable symptoms.

In contrast, while a faulty fuel pump might lead to inadequate fuel supply and subsequently a rough-running engine, it typically would not maintain normal manifold pressure. Improper ignition timing would normally cause a reduction in engine performance across all cylinders and would likely affect the magneto drop as well. Similarly, a blocked air intake would restrict airflow and could raise manifold pressure only until the engine starts to bog down, which would not typically cause a smooth operation of the remaining cylinders. Thus