Originally Posted by JABRONI
After many years of racing I find it interesting that there is no clear answer as to what the mid range needle does for you car.
What I am trying to say is that, You have a low end adjustment, and a High end adjustment that in most cases controls the Temp of your engine, however, I really havent seen any comments on what it typically is the (Brass) mid range setting, other than it is best not to mess with it because the factory settings are supposedly set in stone.
Just wondering if anyone can actually shed some light on what it can be used for and how, if, when to use it effectively ????????????
Here you have it. I can't remeber where I got this article, but it is the best expalnation of howthe so call Mid Range Needle works.
Mid Range Needle
The screw inside the slide body is really the low-end needle, and it plugs into the main jet (the 'brass needle' in the opposite side, usually flush to the surface of the carb body) without obtruding it 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 (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 passes 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. 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 point 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. You should see smoke when you open the throttle at low speed.
Depending at which opening of the barrel the jet (brass 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 jet (brass needle) and readjusting the low-end needle. But be careful screwing the 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 warns you about to not touching this adjustment.
The position of the 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 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 hydro lock.