What effect does a mixture set too rich have on the engine?

When the mixture is set too rich, it means that there is an excessive amount of fuel in the air-fuel mixture being supplied to the engine. This rich mixture can have several impacts on engine performance, one of the most significant being the potential for spark plug fouling.

Spark plug fouling occurs when unburned fuel accumulates on the spark plugs. In a rich mixture, there is more fuel than can be burned effectively during combustion. This leads to incomplete combustion, resulting in carbon deposits forming on the spark plugs. Over time, these deposits can prevent the spark plugs from igniting the air-fuel mixture efficiently, leading to rough engine operation, misfiring, and ultimately engine performance issues.

The other options do not align with the behavior of an engine running on a rich mixture. For instance, a rich mixture does not improve acceleration or increase fuel efficiency; in fact, it can lead to poor acceleration since the engine may struggle to operate effectively. Similarly, it does not reduce engine noise—in some cases, it can cause the engine to run less efficiently and thus could potentially increase noise levels.