You could have the bottom end tune too rich and your idle is open too far which gives it an idle that won't allow you to shut the carb more as it just becomes way too rich and stalls.
Also could be the other way too! make sure you're tuning your bottom needle to your idle gap, not the other way around!
Taking out the venturi (if it has one) visually set the idle gap to about 0.5mm, prime the engine and start it.
Tune the HSN, then set the LSN so it idles high for 20 secs after a fast run, then lower the idle gap if required, then richen the HSN 1hr.
This is your basic tune, tune on the track for performance now.
If you already knew all this then sorry