During the run-up at a high-elevation airport, a pilot notes a slight engine roughness that is not affected by the magneto check but grows worse during the carburetor heat check. Under these circumstances, what would be the most logical initial action?

Explanation

At a high-elevation airport the full-rich mixture is already too rich for the thin air, and carburetor heat makes it richer still, which explains roughness that grows worse with heat but is unaffected by the magneto check. The logical first step is to lean the mixture and check whether the roughness goes away before assuming a mechanical problem; detonation is associated with lean, hot operation, not this symptom.