Hmmm...
First, get a Weber book- Haynes is fine. There are a buncha things that can cause this.
1. fuel pump- I went through hell with my car when I first got it. It had a tankful of crud, and little bits would get into the pump valves. It'd idle, but had no go. A filter before the pump fixed it... weeks of confusion later! A fuel pressure gage is invaluable here.
2. Float level. I don't have the number in my noggin, but if it's off, the carb won't adjust properly. But for your problem, it'd have to be WAY off.
3. Crud. It really sounds like you may have a plugged jet or 2. But the most usual jets to plug are the progression jets (most people call 'em idle, but the idle passageway is really the one you adjust with the screw) I suppose if the float bowl has junk in it, that'll do it.
4. The power jet. If the diaphragm in the top of the carb is broken (or not getting vacuum) then the power jet won't open, and you'll stumble. This usually shows up as an unwillingness to go from fast cruise to WOT, tho...
5. there's a filter in the inlet on some carbs of this body style. If you can unscrew the fitting where fuel enters the carb, do so and check the little mesh screen there...
6. I've had good luck using a cheap 02 meter system. It's great for finding lean spots vs. ignition problems.

hth,
Toby