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You must be very careful when removing welch plugs. They fit over a very shallow depression, and using to much force will damage the jets or passages under them.
They can be removed by driving a sharpened small punch or screw driver through the plug and popping it out.
When installing new plugs, put them in convex (bowed) side out. Use a small object, like a roll pin punch slightly smaller than the plug, tap the center of the plug to slightly flatten it. This forces the plug slightly larger than the opening, wedging it in. Once this is done, seal the edges of the plug with a hardening sealer or fingernail polish.
The symptom of a bad plug will be a leak, in this case, a fuel leak.
Welch plugs do not usually just go bad, they are simply a plug.
If one goes bad, it will usually fall out.
It is difficult to understand without seeing it, but when a carb is built, there are several internal passages that must be drilled. Some of these passages are at odd angles, and/or never come to the outside of the carb. The solution is to drill a 'chamber' that these passages can be drilled into and from. When all drilling is done, these chambers are sealed, sometimes with a welch plug, and sometimes with a small brass 'BB' pressed into the hole.
Changing the welch plug will not fix a carburetor. Removing the welch plug will only allow access to the passages underneath, the passages that normally become clogged with varnish when gas is left to sit and get old.
What is most likely the problem is varnish buildup inside the carb.
As gas gets old, it turns to varnish and clogs up the passageways inside the carburetor, not allowing enough gas to get to the engine.
This condition is cumulative. Every time gas sits, the varnish builds up just a little more, like coats of paint, until eventually gas can not flow. It will not happen overnight, but the symptoms can show up over night.
When this happens, either the engine simply will not start, or it will not run without the choke on (this reduces the amount of air getting pulled into the engine, changing the fuel/air mixture), or it will run but surges.
Another issue that varnish in the carb can cause is that the varnish may not allow the float needle to seal properly against the seat, causing the flow of gas to not shut off when the bowl is full. The result will be gas overflowing the carb and running into the cylinder, and possibly out the air intake. If the gas gets into the cylinder, it will seep past the rings and down into the crankcase. This will be evidenced by your oil level being over-full and/or the oil smelling like gas.
The only 2 solutions are to either replace the carburetor or give it a good, thorough cleaning.
Most people believe that cleaning a carb involves removing the bowl and wiping it out, then spraying some carb cleaner through it.
This is simply insufficient.
To properly clean the carb, you must remove it, disassemble it (making sure to remove all non-metal parts), and soak it in a commercial solvent for several hours. Soaking it overnight is even better.
Then clean all solvent off with a spray type carb cleaner, making sure to get lots of cleaner into every hole and passage there is. Pay special attention to the tiny holes in the bore of the carb, under the throttle plate for the carbs that have these holes. Use lots of cleaner. And make sure to wear safety goggles to avoid getting the over spray into your eyes. There will be over spray.
Dry the carb with low pressure compressed air.
When reassembling the carb, make sure to use a carb kit, when one is available for your carb.
Occasionally, even a good cleaning is not going to be sufficient, and you may end up having to replace the carb anyhow.
When the varnish gets built up inside, adjustment is impossible.