FAA Powerplant Written Practice Test

If a two-stroke light sport aircraft engine runs with the ignition off, what is a likely cause?

1. The fuel mixture is too rich

2. The battery voltage is too low

3. The engine has overheated

4. The spark plugs are fouled

The correct answer is: The engine has overheated

In a two-stroke light sport aircraft engine, running with the ignition off typically indicates that the engine is still able to draw in fuel and air and undergo the combustion process due to compression and its inherent design. When the ignition is turned off, the electrical spark needed to ignite the fuel-air mixture is eliminated. However, if the engine has overheated, it may lead to the fuel vaporizing before it can be ignited properly by the spark plug. Overheating might also cause engine components to deform or fail, leading to incomplete combustion cycles or loss of control over fuel and compression, allowing the engine to continue running momentarily on residual heat or previous combustion heat. While other factors, such as a rich fuel mixture, low battery voltage, or fouled spark plugs, can cause performance issues, they wouldn't as directly result in the engine maintaining operation with the ignition off. Therefore, significant overheating affecting the engine's performance is a strong candidate for this specific situation, as it disrupts the normal combustion cycle and can alter how the engine operates in the presence or absence of spark.