What situation indicates that an engine may be running rich?

When an engine is running rich, it means that there is an excess of fuel in the combustion process relative to the amount of air. This condition can lead to several observable indicators, one of which is black smoke emanating from the exhaust.

Black smoke is a direct result of incomplete combustion, where not all of the fuel is burnt due to the surplus being provided. The excess fuel fails to achieve its ideal combustion, leading to soot and carbon particles being expelled through the exhaust, which manifest as black smoke. This visible smoke serves as a practical indicator of a rich condition in the engine's air-fuel mixture.

Other signs could involve exhaust gas composition, but black smoke is one of the most immediate and noticeable symptoms. It's important to address this condition promptly, as prolonged rich running can lead to engine performance issues, increased emissions, and potential damage related to the accumulation of unburnt fuel and carbon build-up.