I had similar issues when I put my carbs back together, make sure the float bowls are vented correctly, I had two different forms of venting on my carbs, one involved a notched washer to allow air out, as I recall the other carb actually had a vent tube on the top of the float lid where the nut went in, not sure which is correct (but I assume if correct they would have matched), but if not vented you will get symptoms similar to described, the SU pump should not be able to "overpower" the floats and needles in normal circumstances.
Good luck, Greg