Hey R,
As Bully said, most likely there is some restriction in the idle and off idle circuits. This is actually not uncommon after a carb rebuild, if the gaskets are squeezed just a little too much then after time a small sliver of gasket material will break off and slip into a jet or orafice...and we are talking really small, like the tip of a toothpick. That is all it takes to get these kind of problems.
On rare occations I have witnessed the obstruction clearing on a highspeed run, or a few seconds of WOT. Admitedly it is rare, but if you want to try it, get on the highway and pick a gear that will run the engine rpm at 3000-3500rpm. Only for maybe a mile or so, and be careful and aware of other traffic.
If it doesn't help, "open carb surgery" is needed.
Good Luck