One other thing to look at - muffler pressure.  I've seen similar problems in setups where there wasn't sufficient back pressure, particularly at low throttle settings.  Pitts mufflers are notorious for this.  if you have a Pitts muffler in you set-up, you might try plugging one of the outlets and see if you get a better needle response and more reliable low idle.  Also use the shortest length of tubing you can between the fuel tank and the carb, and make sure there are not sharp bends or twists.

Finally, disassemble and clean the spray bar assembly to make sure you don't have a small particle caught in the spray bar.  It may not be large enough to prevent fuel flow at high throttle settings, but it might be restricting fuel flow at low settings.

Brad</p>