Mecanica

How Carburetors Work….
By Phillip Floria
The modern internal combustion engine is merely an air pump; it pumps air in and air out. One of the first things performance enthusiasts learn is to getmore in you have to get more out. Performance exhaust is useless if you don’t increase the air/fuel mixture coming into the engine.

Ok you know that, so you also know bigger has got to be better, abigger carburetor will allow more air to enter the engine along with a free breathing air box or air cleaner. This is true but only when other factors are taken into account like, jetting, and thetype of cams used.

“Venturi Effect” The carburetor, which is basically a large tube or “Venturi”. Vacuum created by the downward stroke of the piston causes the higher pressure in the float chamberto force the fuel/air mixture through the fuel/air outlet passage (or ports) in the carburetor venturi and vaporized, by the fast moving air stream. A needle sits in the hole to regulate the fuelflow. The throttle cable or engine vacuum operates the needle up and down to regulate the fuel in the air stream.

Not all carburetors are as simple as the one above, some have more than one hole fuelto be drawn from. Most all-modern carburetors have an idle speed adjustment screw it is actually a valve; the end of the adjustment screw is shaped like a needle and it regulates fuel flow when theengine is running at low speed.

For higher speed operation a needle valve is attached to the slide, which is operated by vacuum or cable. In cases where the high-speed needle is attached to theslide, the slide is raised and lowered by the throttle cable. When the throttle is open the needle is raised up out of the emulsion tube allowing more fuel to mix with in increased air allowed by therising slide. As the throttle is closed the slide and needle come down reducing air and fuel.

Most carburetors used on modern motorcycles also have a moving flap in the venturi to regulate the...