When people talk about jetting a Motorcycle carburetor and tuning a Motorcycle carburetor , they’re talking about manipulating the carburetor’s 4 main circuits to optimize gasoline delivery and therefore engine performance.
Most motorcycle carburetor circuits are governed by throttle position and not by engine speed. They might adjust the air screw, adjust the jet needle’s clip position or exchanging the pilot (slow) jet, main jet, throttle valve (slide) or jet needle for one of an appropriate size. A perfectly tuned 2-stroke engine/carburetor delivers a 12.5 to 1 air to fuel ratio.
If you look at a Motorcycle carburetor, you will notice a rather large hole going from one side to the other. This is called a Venturi. Air passes into the engine through this hole (Venturi).
As the velocity of the air entering the carb (and then the engine) increases, it’s pressure decreases, creating a low pressure or vacuum in the venturi. This vacuum moves around in the venturi, as the throttle is opened, and sucks gasoline through the different jets in the carb. The gas then mixes with the air going through the venturi.
The way the Motorcycle jets are made causes the fuel to vaporize as it goes into the venturi. Where the jets are placed in the carb and where the jet’s outlet is located in the venturi, determines what part of the throttle opening that jet controls.
The idle jet system (comprised of pilot air jet, pilot fuel jet and pilot fuel screw) controls from 0% to about 25% of the throttle opening.
The throttle valve controls 0% to 35% of the throttle opening. The needle jet and jet needle control from 15% to 80% of the throttle opening and the main jet controls 60% to 100%. This means that when you open the throttle about one eighth of the way open, all of the gas/air mixture going into your engine is controlled by the idle jet.
The jet needle starts to effect things before the effect of the idle jet ends. This is something to remember when working on carbs… everything is interconnected. Change one thing and it will effect other things.