Because the lawn mower is one of the most widely used maintenance tools, occasional problems occur with its engine. Most lawn mower problems are due to debris buildup or age, which can cause it to run poorly, increase gas usage and accelerate engine wear. The carburetor -- the lawn mower part where the air mixes with the gas before entering the engine -- requires occasional repairs due to these issues. Improper airflow to the gas causes rough operation or the inability of the mower to run.
Find the location of the problem. Determine if the problem is related to the carburetor itself or a related item, such as the breather, screen, gas tank or hoses connected to the carburetor. Remove the carburetor and related parts from the engine to troubleshoot each piece. Disconnect the related parts from the carburetor and check them. Cleaning the parts is the most effective form of troubleshooting because most problems are caused by blockage of air or gas to the carburetor.
Look for air-flow problems. Air is needed to mix with the fuel before entering the firing cylinder. The air filter is where the air mixes with the fuel. Check the air filter for debris or sediment. Replace the air filter when it becomes clogged. Adjust the screw that governs the air-to-gas mixture to control the amount of air that enters the chamber. Turn the screw clockwise to reduce the airflow and counterclockwise to increase airflow. This screw can vibrate loose and require adjustment over time. Check the fuel-chamber cap. This cap can become loose from vibration or improper tightening. Examine the cap for cracks or holes. Loose or damaged fuel caps can allow air to leak into or out of the cylinder causing improper airflow. The cap may also require cleaning due to debris or grass clippings clogging the vent holes. This can cause vapor lock, starving the carburetor of fuel.
A variety of issues can cause fuel problems. Look for the gasket on the bottom of the carburetor; if the gasket is missing, worn or dried out, replace it. This gasket helps keep fuel from leaking out of the carburetor. Look underneath the carburetor to determine if the gasket is damaged or missing. Look at the jet at the bottom of the carburetor. This is where the fuel enters the carburetor. If the jet is clogged, fuel cannot enter the carburetor and reach the fuel line to the engine. Blow out the jet to clear it with a stream of water or forced air.
Examine the fuel line and gas tank. The fuel line delivers the fuel from the gas tank to the carburetor. Disconnect the fuel line from the carburetor and hold it upward. Unclogged fuel lines will allow gas to flow through steadily; slow or thin streams of gas are signs of partial blockage. Blow forced air through the fuel line to remove the debris. Check the line for cracks or holes. If the fuel line cannot be unclogged or has signs of damage, replace it. Check the gas tank for damage or sediment. The interior of the gas tank can start to flake away due to age; this can cause sediment in the gas. Along with other debris, sediment can cause improper fuel flow to the carburetor. Cleaning out the gas tank can help remove debris and sediment.
Annual maintenance can help prevent many problems causing the need for repairs. A thorough check of systems and parts related to the carburetor at the beginning of the mowing season can help eliminate repairs later. Not allowing the fuel to run out also helps eliminate sediment because the sediment cannot settle when fuel is present; debris can also enter an empty tank. Using fresh fuel also helps maintain the carburetor and related parts. Old fuel can become abrasive, thereby causing debris and sediment to form in various parts and lines.