Here's a picture of the carb D8 is talking about. If your carb looks like this, the part that's tightened with the allen wrench is meant to be tightened down and left alone. That's not an adjustment. It has O-ring seals as well, but the moment you crack it loose to make an "adjustment," you take one of the O-rings out of the equation and you're sure to have an air leak, big or small. It's simply not designed or intended to be adjusted.
The picture below is of a Sirio 3-needle carb. Note the slotted brass screw in the side of the carb. This IS designed to be adjusted. Both of the O-rings are deep in the bore and they can accommodate movement of the spray bar without leaking (usually). Both carbs use the same body, but one is clearly not intended to be adjusted and the other is.
If your spray bar is not the adjustable type, then you definitely don't need to worry about it, but even if it is, leave it alone. If necessary, adjust it so it's somewhat close to flush as shown here. Make all the low-speed adjustments on the other side of of the carb where the linkage attaches. That's your low speed needle.
Your answer to my last question tells me that your fuel mixture is too lean on the main needle. The reason I ask is, when the main needle is too lean, the engine will run on when you let off the throttle. After a few seconds of running at idle with an OK low-speed setting, the engine will start to get the right amount of fuel and the engine will idle down. So,
start by making the main needle more rich and see what happens. Don't worry if the engine is a little sluggish right after starting - this is normal. When the engine warms up it leans out naturally, so perhaps you tuned it when it was cold or not up to full temperature and it's a little too lean on the high speed needle.
FYI - some carbs have black oxide steel needles and spray bars, and like these, some are made from brass. It makes little difference - they're pretty much exactly the same.