I could be wrong but "screw B" appears to be an air bleed screw, possibly for idle air/fuel mixture adjustment.
Screw A appears to be a throttle slide idle speed adjustment. I have no idea what all of the other hose fittings control.
After trying a Genuine Delorto SHA and a clone Delorto SHA, i found no throttle response or power improvement over the NT, so i went back to the original NT carby as i found it to be more user friendly when jetted correctly.
The most significant improvement was the Walbro style diaphragm carburettor, because it has external jetting screws, making air/fuel mixture adjustment a very simple and mess free job.
A diaphragm carburettor just runs properly and stays clean, as it doesn't suffer from air or fuel leaks.