Webers can only take 3 psi MAX. I run a mechanical holley with a dial type pressure regulator. If you go over 3 psi then you will blow out the seals on the carbs. I actually have mine down around 2 and it works great.
If your pressure is at the correct level there are several other things that will cause what you are having. What chokes, mains, idles, idle holders, emulsion tubes, air correctors and most important bypass jets are you runnning?
Matt
Got to love these carbs