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Depending on the exact carb you have, there are different size plugs. The plugs that come in the kits fit most, but not all the holes. I have had to turn new plugs to fit the holes that match the kit.
The large plug is located just under the front mounting hole. Be super careful drilling it out as you can easily drill right through the keyhole for the idle jet (hope that makes sense). I personally wouldn't drill it out because you can easily get to that passage through the other plug for the idle.
Before doing any drilling I would suggest trying Tom Endy's way of cleaning out the passages with a paper clip.
http://modelabasics.com/Carb/ZenithPassageways.pdf