Why Does My Lawn Mower Run Terrible After It Is Warm & Is Restarted?

Vapor Lock

• Before breaking down and cleaning the fuel system, a few minor problems can also cause the mower to shut off or run poorly after it's warm. Vapor lock occurs when the pressure inside the fuel tank is too high to allow fuel to flow with the pulse of the crankcase into the carburetor. When the fuel gets heated, during normal operation, a vent on the fuel cap allows the pressure to stabilize and the gas to keep flowing. If the vent of the fuel cap is plugged from dirt or grass, the gasses can't vent. Clean or replace the fuel tank cap.

Plugged Fuel Passages

• Another common problem that can instantly shut off the fuel flow into the carburetor happens when the fuel passages get blocked. Dirt, grass or other material can fall into the tank during a refueling stop. When the engine starts running, the gas will suck the material against the fuel filter or hoses and shut off the fuel flow. Drain any gas from the fuel tank and check for any dirt or material in the tank. Clean the fuel tank if it's dirty and pull out the fuel filter and replace it.

Plugged Air Passages

• If the engine is primarily just running poorly, but still running, usually enough gas is getting to the carburetor. The problem likely exists in the ratio of air to fuel, which is needed for proper combustion. The air filter purifies all incoming air and the muffler vents out all heated gases. If these two air points get blocked, as the engine warms up, requiring more incoming and outgoing air, the engine will shut off. Clean the air filter and muffler to make sure air passages are clear.

Improper Carburetor Settings

• The carburetor will also regulate the inflow gas and air into the cylinder. If this adjustment is off, even a little, the fuel flow will get disrupted and the engine will shut off as it requires more air and fuel to continue working at full speed. Adjusting these screws requires a soft touch, as even minor adjustments can have big impacts on the carburetor and engine. Most lawn mower manufacturers recommend a service professional adjusting your carburetor.