Dellorto carb problem

Hi

Trying to set the idle screws on the carbs the other day, and I adjusted all
of them apart from one. The idle screw for number three cylinder does not
affect the engine at all, no matter how far I screw it in or out. I checked
the idle jet, which I thought maybe blocked, but it is fine. What could the
problem be?
I have also noticed when the engine is idling, it sounds as though it is
missing, and it is the same under light throttle. But when I put my foot
down it picks up and goes great. Have I got a blockage somewhere in the
idle circuit? What can I do to remedy this?

Cheers