[Marshall V. Daut]

Years ago, I chased a pesky leak from a Zenith for weeks, going through all you have done. A very frustrating experience. It turned out that there was a crack in the body radiating from the main jet hole. The fuel was leaking past the brass plug and gasket through that tiny crack that was almost impossible to see. I had thought the fuel was coming from the carb's intake throat and was running down to the lowest point on the carb = the main jet plug. I deduced that air rushing past the carb while driving was forcing fuel to migrate towards the open intake throat, dripping from there once the car was stopped. I had been looking in the wrong spot. The only way I found the crack was to glass bead the lower body AGAIN and inspect the result closer. And there it was! Apparently in an effort to stop a leaking carb, someone in the past had over-tightened the main jet plug, cracking the delicate metal. The crack started inside the hole just below the plug and was even in the threaded portion. It then spread outwards in the lower casting, which is where the gasoline was leaking. It was faint, but the lower casting definitely was cracked. I replaced the lower body and transferred all the new and checked-out parts to the new carb half and the leaking problem was solved.
Check that your lower casting isn't also cracked in that area. Dripping fuel from the carb's throat fooled me into thinking the problem lay there. Who'da thunk that the lower casting was hiding a hairline crack? That's only example I have ever seen of this. And of course, it happened to ME!
Marshall