Why Does My Lawn Mower Not Run When It is Hot?

Vapor Lock

• The fuel tank cap contains a small, pin-sized hole or holes in the top. These holes allow heated air inside the fuel tank to escape. If these holes get plugged, the engine will undergo what is commonly referred to as vapor lock. Vapor lock occurs when the pressure inside the gas tank is greater than the pressure inside the carburetor. When this happens, gas stops flowing to the carburetor and the engine won't run once the heated gasses in the tank get too hot.

Clogged Air Filter

• The air filter, which sits directly above the carburetor, purifies any particulate matter from the air before it can enter the carburetor and the engine. With so much dust and grass floating around the lawn mower's engine, the air filter really needs to work hard. If this filter gets too dirty, it will clog up the incoming air and, as the engine heats up, the incoming air won't be sufficient to provide enough air for the fuel mixture. When this happens the engine may even start smoking and will shut off.

Clogged Spark Arrestor

• On the opposite end of the air system, the muffler regulates the venting of the burnt up gasses leaving the combustion chamber. Since some of these gasses are still burning as they leave, a metal screen sits inside the muffler to prevent these embers from leaving the muffler and starting a fire. These screens, called spark arrestors, are required by law in most areas. However, they can clog up and shut the engine off when it gets too hot.

Carburetor Problems

• The carburetor inside a lawn mower brings fuel into the engine, mixes it with air and then sends it to the combustion chamber. As the engine heats up, it will require more fuel coming through the carburetor to keep the combustion process going. If the carburetor is dirty or clogged, the fuel supply will be insufficient and the engine will shut off. Lawn mower carburetors need regular cleaning, maintenance and adjustment, but it's better to let a professional perform these tasks, as the beginner can do serious damage to the engine.