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Working with Welch plugs
Engine performance problems often can be solved by paying attention to a simple thing called a Welch plug.
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A Welch plug is a slightly cupped disk of soft aluminum or steel. It is placed into a circular hole that is just slightly larger than the disk, with the cupped surface upward.
You tap the center of the disk with a flat punch and the cup flattens out, pushing the plug’s outer edge tightly against the walls of the hole. Be careful. The plug must end up flat; if you hit too hard and push the plug’s center too deep, it will be cupped again and it won’t seal.
You can add a bit of sealer, like varnish or shellac, to fill in tiny imperfections in the seal and help ensure the plug won’t vibrate out.
These tiny imperfections in the Welch plug’s seal are rare and often overlooked when troubleshooting an engine that runs rough or lacks power.
After you check all the usual suspects like ignition, fuel filters and carburetor adjustments, check out the Welch plugs that are often used on small-engine carburetors to seal air and fuel passages that run through the carburetor body. A plug leak, no matter how small, can cause the carburetor to run too lean (when excess air sneaks in) or too rich (when fuel flow is disrupted). Corrosion on the inner face of a Welch plug can break loose and clog passages. Some carburetors on chain saws and string trimmers use a Welch plug to cover a passage that contains a filter screen. To clean the screen, you must remove the plug. On Tecumseh carburetors, a Welch plug on the side covers a critical chamber that feeds fuel into the incoming air stream. The plug for this chamber is included in every service kit, and you really can’t do a thorough rebuild without removing and replacing the plug to check for clogged passages inside the chamber.
Be very careful and follow the service manual when removing a Welch plug. You’re often told to use a tiny chisel to punch through the plug and pry it out. Punch too deep, and you can ruin the carburetor by distorting the body and punching off tiny internal passages.
You’ll also find Welch plugs on engine blocks, cylinder heads and other large castings. As with their small cousins, these large plugs can cause strange behavior and are worth a look when solving problems.
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