An engine exhibits hunting and surging at idle and top no-load speeds but not under moderate loads. What is the most likely cause?

When an engine exhibits hunting and surging specifically at idle and top no-load speeds, it suggests that the air-fuel mixture is unstable at those operating conditions. The correct identification of the idle circuit's air bleed being blocked plays a crucial role in this situation.

The idle circuit is responsible for providing the correct mixture when the engine is running at low throttle positions, such as idle. If the air bleed in the idle circuit is blocked, it will restrict the airflow necessary for mixing with the fuel, leading to an imbalanced mixture. This imbalance manifests as fluctuations in engine speed, known as hunting and surging, primarily because the engine cannot maintain a stable idle due to inadequate fuel aeration.

In contrast, under moderate loads, the engine’s airflow dynamics change, enabling it to run more efficiently even if the idle circuit is impaired. This is why the issue is not present under those conditions. Hence, the blocked idle circuit air bleed is a plausible and reasonable explanation for the observed symptoms at idle and top no-load speeds.