What is the correct air/fuel ratio for an engine running at idle?

When an engine is running at idle, it typically requires a richer air/fuel mixture compared to higher operating conditions. This is because at idle, the engine operates under low loads and lower speeds, which can result in incomplete combustion if the air/fuel mixture is too lean. A richer mixture helps ensure that there is enough fuel present to keep the engine running smoothly, maintain combustion stability, and reduce the risk of stalling.

Additionally, a rich mixture at idle can accommodate any variations or inconsistencies in fuel delivery and air intake, ensuring reliability and performance. In contrast, if the mixture were too weak, it could lead to difficulties in starting, rough idling, or even engine stalling.

In summary, an idle condition generally favors a rich air/fuel ratio, which optimizes engine performance and ensures effective combustion under these low-load situations.