afm

Following Uriah's explanation, I thought these one might add some more light to your question....hope it helps

AFM

Mid Range Needle

The screw inside the slide body is the low-end needle, and it plugs into the main jet, or so called mid range needle (the 'brass needle' in the opposite side, usually flush to the surface of the carb body) without obtruding it completely, to let a little bit of fuel to pass when the needle plugs into it to allow the engine to idle.

The dimensions of the main jet, or mid range needle (brass needle), its 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 more air to pass, in fact, you're moving the low-end needle too, allowing at the same time more fuel to pass too. The low-end needle adjustment is critical, because the engine relies on this adjustment for its temperature control when idling in the infield of the race track (This is the zone where the engine does 90% of its work). If the top speed of the engine is good and the engine runs hot, you should generally richen this needle (open). You should see smoke coming out of the pipe when you open the throttle at low speeds.

Depending at which opening of the barrel the main jet (mid range needle) is totally uncovered, and the carburetor starts being mandated by the adjustment of the high-end needle, and how much fuel this last adjustment permits to pass, rather than being governed by the low-end needle, can be varied (and the power band of the engine too, but slightly) by screwing or unscrewing the main jet (mid range needle) and readjusting the low-end needle.

But be careful, screwing the main jet too further into the venturi 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 they warn you about not touching this adjustment, because it comes factory tuned to it’s ideal average.

The position of the main jet, relative to the venturi also changes the position on where and how much the vortex of air that enters into the carb varies how finely the fuel is sprayed into the air that enters (fuel is converted into a mist to be burned), a drop, no matter how little it 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 hydro lock.