The carb in the photo is not a Dellorto - it's a Gurtner.
Gurtner carbs (at least this type) do not have an idle mixture screw; the screw you picture is the idle speed screw. It affects idle speed, and nothing else.
There should be a black rubber gasket (like an o-ring, but flat) inside the mounting flange of the carb. It helps to seal the joint between the carb and manifold. They are frequently missing.