The 'brass screw' isn't a needle. Is the main jet of the carburetor. After this lines, there's a complete text describing how our carburetors work. Seems that many people gets puzzled with all the needles and how to adjust them. I suppose a minimum knowledge on where's the high-end needle, low-end needle, spraybar or main jet and idle speed screw.

The screw in front to the 'brass screw' (located at the side of the carburetor and usually flush to the surface of the carb body) is really the low-end needle. And it plugs into the main jet without obstruding completely to allow a little of fuel to pass when the needle plugs into it to allow the engine to idle.

The dimensions of the jet, their position relative to the venturi, and the dimensions, ramps and shape of the low-end needle dictates the fuel curve of the carburetor (or how much fuel enters by each cfm of air drawn into the engine at a relative carb opening).

At idle speed, the engine is controlled by the low-end needle and how much air pass into the engine via the position of the barrel. But when you start to move the barrel to allow to pass more air, in fact, you're moving too the low-end needle, allowing at the same time to pass more fuel too.

Depending at which opening of the barrel, the jet is totally uncovered and the carburetor starts to being mandated by the adjustment of the high-end needle and how much fuel permits to pass this last adjustment rathern than being governed by the low-end needle. This can be varied (and the powerband of the engine too, but slightly) by screwing or unscrewing the jet and readjusting the low-end needle. But be careful screwing the jet too further into the ventury can lead to mix and temperature problems (in fact, you're leaning the mid rpms, where the engine operates at partial opening of the carb, this can lead to problems and erratic operation) this is why on almost all the engine booklets warns you about to not touching this adjustment.

The position of the jet, relative to the venturi changes too the position on where in the vortex of air when enters into the carb and varies too how much or how finely is the fuel sprayed into the air that enters (fuel is converted into a mist for being burned, a drop, no matter how little is doesn't burn and can create many and serious problems). This last can lead up to a conrod breakage (common) or piston breakage (not common but seen some) due to hydrolock.