The issue was not the tap (I've run nothing but fast flow taps for years). It's a si24/24 with about a 130 main jet. The passage from the float bowl to the jet stack was too small. I drilled it out to 2.5mm and that fixed it immediately. If you run a main jet larger than about a 125, you will see this problem on undrilled si carbs. I thought it was already drilled, so was surprised when I looked at it and realized it was still stock.