You've gone thru everything pretty thorough. The problem must still be in front of you, not fuel pressure if you have regular mechanical pump.
I would have a looksee at the metering block gasket, front bowl. The early 1850's have a crossover tube (internal) for the accelerator pump circuit just above the power valve, this is a little short brass tube with an 'o' ring on each end. You don't have one in your carb, but! the gaskets for that type of metering block are included in the rebuild kit, it has a half-moon notch cut out of it just above the opening for the powervalve well. If you use that gasket with the modern 1850 fuel will run from the accelerator pump circuit (there is enough of a pressure differential) directly into the engine thru the vacuum hole in the bottom of the baseplate (where vacuum is present for the power valve). It is one of those problems where you can't see anything wrong and it'll run ok at highway speed but it'll idle stupid and the idle screws will be ineffective. If i were more computer literate i could provide you with the pics of each gasket, again, the wrong one has a half-moon cut out above the toumbstone-like opening for the powervalve chamber.