What is a common sign of an excessively lean fuel mixture?

An excessively lean fuel mixture occurs when there is not enough fuel in proportion to air entering the engine. One of the most common signs of this condition is engine overheating. This happens because an inadequate fuel supply reduces the cooling effect that fuel has as it vaporizes and combusts. In a properly functioning mixture, fuel helps absorb heat produced during combustion. When the mixture is too lean, there is insufficient fuel to provide this cooling effect, which can lead to elevated engine temperatures and, ultimately, engine damage if not addressed.

While reduced engine performance can be a result of a lean mixture, it is not as direct an indicator as overheating. Similarly, black exhaust smoke indicates a rich mixture, which is the opposite of a lean condition, and increased fuel consumption typically suggests a richer mix rather than a lean one. Thus, engine overheating stands out as the hallmark sign of an excessively lean fuel mixture.