paragliding training center
by Had Robinson
These notes are for the Walbro carburetors. Others are similar.
Removing the welch plugs can be done with the Walbro 500-16 tool or equivalent. However, this tool is expensive ($33 or more) and requires skill in order to not ruin the welch plug well and the tiny passages beneath it.
If you happen to have this tool, hold the punch at a shallow angle to the welch plug and drive it (!) through. Of course, if you do not do it correctly, you have ruined the carburetor. The drawing below (from Walbro) shows the correct angle.
A much better (and safer) way to remove welch plugs is to use a spare drill chuck (or take one off your electric drill), a 3mm (1/8") drill bit, and a 3mm (1/8") punch. Sets to remove welch plugs in carburetors are available but they are also expensive ($70-150) and contain many more tools than are needed to service the Walbro carburetor. If you take the chuck off your electric drill, note that the center screw that keeps the chuck from unscrewing has a reverse thread.
For installing the plug, you will need a punch, rod, or bolt that is the same diameter as the welch plug (8mm for the WG-8).
Here is a welch plug removal kit consisting of a 1/8” double-ended bit, a bit holder, and a special 1/8” pin punch that has a 45º tapered end. Unfortunately, I do not know where I purchased this set and searching for it on the Internet has not produced any leads.
Drill a hole by hand in the center of the welch plug using the chuck/holder and the bit. YOU CANNOT USE A DRILL PRESS OR AN ELECTRIC DRILL TO MAKE THIS HOLE! If you do, you will likely (and suddenly) drill through the plug and then into the guts of the carburetor, ruining the tiny holes in the welch plug well. The hole must be drilled by hand, slowly and carefully. Apply steady pressure until the bit starts to penetrate through the plug. At this point, pull up on the bit just enough to keep it from bottoming out in the well. It is relatively easy to cut through aluminum with a sharp steel bit.
Once the hole is made, push the pin punch just into the hole and move the punch to one side. The plug will come right out.
After the plug is removed, thoroughly clean the hole with compressed air. The best way to do this is to use a very narrow tubed compressed air tool or a can of compressed air used for cleaning computer keyboards. Put the tube in a corner of the well where there are no holes, lightly press down on the tube and apply air. The air will rush over the surface of the main hole (but not directly down the tiny holes) and it will remove the bits of aluminum from drilling the hole in the plug. You do not want to blow any bits of metal down the holes going into the carburetor throat.
After this, clean the carburetor with compressed air and carburetor cleaner. Examine the carburetor carefully with a magnifier. You must be sure it is absolutely clean and free of any debris. The tiny holes in the idle circuit can be visually examined with a bright light to be sure they are clear. Do not use any object to clean the holes because it is very easy to ruin the holes by changing their size with a piece of wire, for example.
Use a punch, rod, or bolt that is the same diameter as the plug (8mm for the WG-8). (If you use a bolt, you must be sure that the end is flat.) Coat the rim of the plug with nail polish and set the plug in the well with the convex surface facing up. Place the punch, rod, or bolt on top of the plug and tap it gently with a small hammer making the convex surface of the plug flat. This seals the plug. The plug must be flat or it may interfere with the operation of the metering lever diaphragm. Wipe off any excess nail polish. If the plug leaks, the idle mixture needle will not work well.
The finished welch plug. When the convex surface is flattened, the plug expands and tightly seals the well.
Reassemble the carburetor, adjust, and test.