Which condition would indicate a general weak-engine condition when tested with a fixed-pitch propeller?

A general weak-engine condition can be indicated by lower than normal static RPM at full throttle operation. When an engine is operating properly, it should reach a specific RPM as specified by the manufacturer when the throttle is at full open. If it falls below this expected RPM, it suggests that the engine is not producing the expected power output. This could be due to a variety of issues such as inadequate fuel supply, poor ignition performance, or mechanical problems within the engine itself, all of which result in lower performance.

In contrast, the other scenarios do not necessarily indicate a weak-engine condition. For example, higher manifold pressure at idle RPM compared to static RPM could indicate an inefficient airflow or problems with the restriction in the induction system, but not necessarily a weak power condition. Similarly, higher than normal manifold pressure for any given RPM might reflect an engine that is producing more power than anticipated, which would not align with a weak performance. Lastly, a static RPM greater than cruise RPM could indicate that the engine is producing sufficient power, possibly due to variable factors like propeller pitch affecting performance in different operating conditions. Thus, the testing of static RPM serves as a reliable indicator of overall engine health and performance.