There are three basic types of carbs: updraft, downdraft and sidedraft. Those terms refer to the direction of airflow through the carb’s air horn. A downdraft, the most common automotive type, mounts on top of the intake manifold and air flows down through the barrel and into the engine. And updraft, as you’d expect, mounts under the manifold and air is drawn up through the barrel. Many small engines use sidedraft types.
No matter which way air flows through it, a carb must blend that air with fuel to create an explosive vapour. The ratio of atomized fuel to air is essential in creating the correct blend, allowing the engine to run properly. Liquid gasoline won’t ignite properly or at all in the cylinders. If there is too little air, the mixture is said to be “lean.” Too much fuel creates a “rich” mixture.
Strictly speaking, a “Lambda” fuel ratio (14.7:1 air to fuel) is required for a complete fuel burn. But engines run best on varying ratios depending on their operating speeds. For example, a richer mixture is required for starting, because fuel doesn’t vaporize well in a cold engine and the speed of air moving into the cylinders is relatively slow. Closing the choke valve on a carb creates that rich condition and allows the fuel-air blend to lean back out as the engine reaches operating temperature.