Understanding After-Fire in Kubota 2020 Gasoline Mowers: Causes and Solutions

When operating a Kubota 2020 gasoline mower, it is not uncommon to experience a phenomenon known as after-fire, which can often be mistaken for backfire. After-fire is the occurrence of sparks or flames immediately after the ignition has been turned off, typically after the engine has stopped rotating. This can happen up to a few seconds after the engine has stopped, which is different from a backfire, a more violent and sustained burst of flame that can occur during the stopping process.

Physics Behind After-Fire

After-fire is a result of the physics involved in the internal combustion process of a gasoline mower. When you switch off the ignition, the spark is cut off, but the engine continues to run on inertia for a short period. During this time, the pistons continue to move up and down, and valves continue to open and close. The air-fuel mixture keeps being drawn into the combustion chamber because the engine is still rotating.

This continued rotation means that the fuel-air mixture can still ignite without the spark, leading to the after-fire phenomenon. This typically only occurs under certain conditions, such as when there is a remaining spark or hot carbon on the piston. The fuel mixture can ignite, causing a sudden burst of flame or sparks, which can be perceived as a backfire by the operator.

Solutions to After-Fire Phenomenon

One of the simplest and most effective solutions is to allow the engine to idle for a few minutes after turning off the ignition. This allows any remaining fuel-air mixture in the cylinders to burn off naturally, minimizing the likelihood of after-fire. If you observe that the mower still produces a popping or backfiring sound even after idling for a while, there could be another issue at play.

Exhaust Valves and Carburetor Malfunction

Another possible cause of after-fire is a sticking exhaust valve. In some cases, the lack of a spark can lead to the valve staying open longer than intended, allowing fuel to escape into the exhaust system. This can result in a popping or backfiring sound when the engine finally comes to a complete stop.

Additionally, many engines have a solenoid that blocks the carb jet when the ignition is turned off. This solenoid is typically located at the bottom of the carb bowl and is designed to prevent backfire by blocking the fuel supply immediately when the engine is turned off. However, if the solenoid is malfunctioning or the carb jet is not closing properly, it may contribute to the after-fire phenomenon.

To troubleshoot, it is recommended to:

Allow the engine to idle for a few minutes before shutting off Check for any sticking valves or malfunctioning carburetors Inspect the solenoid and ensure it is functioning correctly

By understanding the causes of after-fire and taking appropriate steps to address them, you can ensure smoother operation and more reliable performance of your Kubota 2020 gasoline mower.