Dang - thanks for the power valve explanation. It appears I was mistaken about the way the power valve functioned. I now understand it is "on" unless it experiences a certain amount of vacuum.
Nonetheless, I believe its ability to function properly depends not only on having a functioning diaphram, but also on the fuel level in the bowl, and thus on float settings.
I've looked at this only briefly, but I think a high level of fuel in the bowls can diminish the effect of engine vacuum on the valve.
If I am right (and even if I wrong about this, proper float settings can't hurt), making sure your float levels are correct should be the next step.