Which statement is true regarding fuel mixtures in air cooled reciprocating engines?

In air-cooled reciprocating engines, the fuel-air mixture behaves differently depending on the engine's operating conditions. At idle, the engine is operating at low power and therefore requires a richer fuel mixture to maintain stable combustion, as there is less air entering the engine. This richer mixture helps prevent engine roughness and misfire when operating at low RPM.

Conversely, at rated power, the engine typically runs more efficiently with a leaner mixture. This is because the air density is higher, and the engine can effectively use a more precise air-fuel ratio for optimal combustion. However, it's important to note that the mixture is still enriched compared to idle, but not as rich as in idle conditions, allowing for maximum engine output without fouling.

Therefore, both statements can be considered true when viewed in the context of how air-cooled engines operate at different load conditions. The mixture at idle is indeed richer than at rated power, while the engine at rated power uses an optimal mixture that is less rich than idle but still replete enough to support power requirements. This nuanced understanding reinforces the correct answer, which recognizes that both conditions have validity in the context of fuel-air mixtures in these engines.