Howie!
El DIablo (and FlatironMike) is right about checking your throttle linkage, as the springiness of the coiled sheath may be working against the action.
And he was right that a worn Linkert can also develop these symptoms, particularly from not only wear at the throttleshaft, but warpage of the mating flanges of the carb and manifold.
But his suggestion of using brake cleaner will not display anything but enormous leaks, nor will it pinpoint the location. It will also depend upon RPM, temperature, and other variables.
An absolute, static test that will pinpoint even the tiniest of leaks is simple airpressure and soapy water:
http://virtualindian.org/11techleaktest.html.
Correcting the throttleshaft wear is simple replacement with fitting of new bushings;
Correcting the flanges requires lathe-cutting the carb's flange, and grinding
the manifold's flange back to "flat".
Correcting leaks between the manifold and cylinder can be more complicated. Modern compliant seals from the material called "PEEK" are ideal replacements
if the manifold spigots are pristine.
Correcting leaks between the intake nipple and the cylinders however, are major surgery, as discussed in the previous link.
Good luck, we hope it is just your control coil!
....Cotten
PS: Sinner!
He mentioned in his subject that it is an 80" Flatty. The last were produced in '42, although U's made through '48 could be overbored to 80". H-D did not introduce the auto-advance until '64.
The main nozzle has nothing to do with idle.
