There is a small brass plug (approximately 3/16 inch in diameter) in the underside of the upper housing of Late model carburetors, just forward of the small brass fixed idle jet. The basic purpose of these plugs is to close off the bottom of this particular idle passageway in the upper housing after it was drilled, similar to the way 1/4 inch brass plugs are used to close off the ends of the oil passageways in the block.
At some point, Zenith began drilling small bleed holes in these plugs so as to allow a small amount of air to be drawn in to mix with fuel passing through the idle passageway. Since these bleed holes varied so greatly (from “0” to .057″), I originally had the idea that they can’t be too important.
More recently, I drilled a small hole (about .040″) in one of these brass plugs in a carburetor which had no hole previously, in an effort to improve poor idle performance. After drilling the hole, I couldn’t even get the engine to idle at all – not even poorly.
From this experience, I conclude that whether or not a carburetor has a bleed hole in this plug, as well as the size the size of the hole if it is present, is somehow linked to the internal configuration of the idle passageways of that particular vintage of carburetor, and should not be altered in the field.
If you have a copy of Newsletter 26 (February 7, 1997), or if you bought our Newsletter Compendium, you can read more about these bleed holes on page 130 and 131.