Originally Posted by kakluote
anyone has idea on mixture control valve,what does it used for ,and when should adjust it?
The Mixture Control Valve, is actually the main jet of the carburator, commonly called mid range needle.
The dimensions of the this main jet (brass needle), its position relative to the venturi 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 more air to pass, in fact, you're moving the low-end needle too, allowing at the same time more fuel to pass too.
Depending at which opening of the barrel the main 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 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 itís 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.
Hope its clear