There is an old-guy's trick that you could try. Once upon a time I bought an old 850 Mini in lousy condition. It very slowly flooded its SU carburetor at idle, but was fine (for a tired old Mini) when in motion. I fitted a new needle and seat, with no effect on the problem. I therefore swallowed my pride and went to see an old guy who ran a carburetor repair shop. He took the carburetor top, turned it upside down, slurped on the fuel-line connection, then blocked it with his tongue, waiting to see how long it took to lose the vacuum by leakage. It lost the vacuum fairly quickly, and seeing that it was a new needle and seat, he unscrewed the seat from the body and reinstalled it with liquid sealant on the thread. After that it would hold vacuum properly. Didn't flood either.
My point in this long story is that you could make the same test, and find out whether your needle and seat will hold vacuum. If it doesn't you can probably find out why by doing other tricks, such as sealing each possible leakage path in turn with a drop of oil.
