An engine that "hunts" or has unstable rpm is most likely to have:

An engine that exhibits a condition known as "hunting" or unstable RPM is often experiencing fluctuations in its speed due to an imbalance or inconsistency in its operation. The most direct cause of such instability in RPM is commonly associated with a large air leak in the intake system. When there is an air leak, the engine can draw in an unintended amount of air, disrupting the air-fuel mixture that is critical for consistent combustion. This change causes the engine to struggle in maintaining a stable RPM since the fuel mixture becomes leaner, leading to irregular engine behavior.

While improper ignition timing and incorrect spark plug heat ranges can certainly affect engine performance, they are less frequently the primary cause of hunting compared to an air leak. Ignition timing issues might lead to rough running or misfires, while spark plug heat range concerns often manifest through excessive fouling or detonation rather than a direct and immediate hunting situation. Therefore, in the context of engine instability specifically characterized by rapid changes in RPM, a large air leak is the most likely reason for the behavior observed.