In a reciprocating engine equipped with a float type carburetor, why might the engine run rich only at idle RPM?

In a reciprocating engine with a float-type carburetor, the engine may run rich at idle RPM due to a leaking needle and seat. The needle and seat assembly controls the flow of fuel into the carburetor bowl. If there is a leak, fuel will continue to enter the carburetor even when the engine is not demanding more fuel, such as during idle. This results in a richer mixture because excess fuel accumulates in the carburetor and is delivered to the engine.

At idle RPM, the engine requires less fuel than it would at higher RPMs. However, if the needle and seat are not sealing properly, the fuel supply can exceed the engine's needs, leading to a rich mixture that can cause poor engine performance, rough idling, and increased emissions.

The other options, while they may affect engine performance, would not specifically cause a rich condition solely at idle RPM in the same manner as a leaking needle and seat. Improper ignition timing would affect combustion efficiency more broadly, while a weak fuel pump could lead to inadequate fuel delivery overall rather than a rich condition at idle specifically. A dirty carburetor could lead to general performance issues and may cause running rich across a range of RPMs, not just