Welch plugs cover carburetor passages that affect the performance of a lawn mower's engine system. Welch plugs help ensure that proper amounts of air and fuel circulate within mower engines to keep them running smoothly. Sometimes mower performance problems can be traced to a faulty plug seal or a missing plug.
Welch plugs are small, cupped disks that are usually made of aluminum or steel. Their sizes vary, but a lawn mower may have just one plug with a diameter that measures less than 1 inch. Welch plugs seal air and fuel passages running through a lawn mower carburetor. A carburetor controls an engine's airflow to regulate the amount of air pressure entering the engine. Air pressure affects the amount of fuel drawn into the engine during its operation.
Faulty Plug Seals
The passages drilled into lawn mower carburetors must have tight seals. Welch plugs are designed to seal those passages to prevent leaks that would allow too much air into a carburetor at one time and disrupt the flow of fuel. Sometimes a leak in a plug's seal causes an engine to lose power or run rough. The vibration of a lawn mower also can cause a plug to fall out if it's not tightly sealed.
Replace a missing Welch plug to avoid hampering a mower's performance due to unregulated air and fuel flow throughout the engine. Put a replacement plug on top of the opening it will seal, and use a flat punch to tap the top of the plug's rounded surface until it flattens. The goal is to push the outer edges of the plug firmly against the walls of the opening to get a tight seal. Avoid pushing the plug too deeply into the opening, because the plug will bend inward and pull away from the edges.
Welch plugs sometimes need to be removed from carburetors to clean clogged passages beneath them that are hampering a mower's performance. Removing a plug usually requires driving a screwdriver or sharp punch through the plug and prying it out. However, you could damage the carburetor in the process if you're unfamiliar with removing a Welch plug. Consider taking your lawn mower to a reputable repair shop if you're unfamiliar with cleaning or repairing carburetors.