FAA Powerplant Written Practice Test

What does a rich fuel mixture usually indicate for engine performance?

1. Increased fuel efficiency

2. Higher power output

3. Lower combustion temperatures

4. Risk of fouling spark plugs

The correct answer is: Risk of fouling spark plugs

A rich fuel mixture is characterized by a higher proportion of fuel relative to the air in the combustion process. This condition has specific implications for engine performance. When an engine runs with a rich mixture, it often leads to incomplete combustion. This can result in unburned fuel, which can accumulate and cause fouling of the spark plugs. Fouled spark plugs may lead to misfires, rough engine operation, and reduced performance overall. Therefore, the risk of fouling spark plugs is a significant consideration for engines operating on a rich mixture. In contrast, a rich mixture does not typically indicate increased fuel efficiency, as more fuel is consumed without a corresponding increase in power or efficiency. While a rich mixture may result in higher power output under some conditions, it also can lead to inefficiencies and lower combustion temperatures due to the excess fuel that does not completely burn. Consequently, understanding the consequences of running on a rich fuel mixture is crucial for optimal engine performance and maintenance.