You are going through the exact same thing I am right now with my 72 CB500. I have synced my carbs, adjusted my tappets, timing is perfect, cleaned and gaped points. Everything is pointing to the float needle that seems to get stuck a lot and overflow the float bowl. I have been advised that this is causing a rich running condition and would require a good carb cleaning and it wouldn't hurt to get new float needles, they are cheap.
I never thought that the restrictive nylon connector could be a problem, it might contribute a bit but I don't think it will cause what you and I are experiencing.
I would look into sonic carb cleaning, and get new float needles and adjust float height. at the very least clean your carbs with compressed air and carb cleaner, and soak the needles in carb cleaner for a while.