[GraVitY]

I've rejetted before and I can't help but think how these actually work. I was under the impression that the mains & pilots actually put gas into the cylinders but then I read somewhere that the pilot is for air not gas, which makes me think that the main is for air too right? Looking at the carb float bowl: Gas goes into the bowl and stops when the float reaches its max and shuts off the gas. Looking at the bowl the gas never reaches the main or the pilot jets as they are set higher than the gas level, which leads me to believe that they are put there for air not fuel. And, if so how does the gas then get into the cylinders, from the air/fuel mixture screw?
It's funny actually how something can be so complex and work so well that with time and inovation come up with something even more simple as the fuel injector.

Grav.

 

[Guest]

Carbs work by taking advantage of Bernoulli's principle. Basically, when the air in our carbs passes through a restriction (the venturi) it's pressure drops and it's velocity increases.

Since the float bowl is open to atmospheric pressure this pressure differential draws the fuel up from the float bowl and into the air stream. Both the main and the pilot circuits operate this way, but the pilot jet is smaller for more precise metering at lower air flow levels.

The jets are just above the level of fuel in the bowl so that the fuel is atomized as it's drawn up.* Float level too high and the mix goes rich and blubbery since you're trying to burn raw gas (doesn't burn unless it's mixed with air ~14:1). Float level too low, and the pressure differential won't be enough to pull the fuel up, resulting in a lean mix (low power, high temps).

The idle mix or air bleed screw uses air flow from upstream of the venturi (higher pressure, lower velocity) to draw a small amount of fuel into the air stream. The screw that regulates this circuit can control either the air flow or fuel flow, depending on the design. If it's a fuel screw, turning it CCW will allow more fuel flow richening the mix. If it's an air screw turning it CCW will lean out the mix by adding more air.

We've been messing around with carbs since forever (100+ years), and it's a pretty well refined piece of hardware now. But the earliest carbs were nothing more than drip feeders; some even dropped fuel onto a hot manifold to atomize it!. By comparison, modern all-electronic, computer controlled FI has only been around a few decades (although FI itself is as old as the carburetor) and is still a work in progress.



http://auto.howstuffworks.com/frame...0.3/motoprof/moto/mcycle/carb101/carb101.html

*I need to check this based on andyauger's subsequent post. Atomization takes place in the venturi, although some carbs use emulsion tubes to begin mixing earlier.

EDIT: added link and foot note.

 

[andyauger]

No air bleed jets

Some carburetors have air circuits in them, the SV does not. All the circuits are covered by fuel in the fuel bowl (if the float level is correct and you've got gas). All the circuits pass liquid fuel into the throat of the carburetor where the combination of velocity and exiting a small orifice helps atomize the fuel so it evaporates easier into vapor before entering the cylinder. The idle mixture screw (the one you have to uncap to adjust) operates at closed-throttle position. As the slide rises the needle in the bottom of the slide begins to add fuel. As the slide rises further the main circuit comes into play. There is a good amount of overlap between the three circuits.

Raising the needle in the slide makes the passage bigger at lower slide positions, richening the mixture. Bigger mains pass more fuel than smaller mains. The idle jets should be set for good idle mixture.