All gasoline engines need to burn fuel in order to operate. Contrary to popular belief, liquid gasoline does not burn — only vapor burns — so the liquid must be converted to vapor before it enters the combustion chamber. Gas-powered engines need to run with an air-to-fuel ratio somewhere between 9:1 and 16:1, depending upon temperature, speed and load. In new cars, fuel injection systems do this work but for the first 75 years (or so) of the last century, the carburetor was the device that provided fuel vapor to the cylinders.
Many people think carburetors are hopelessly complicated and impossible to work on, but this is because they don't understand the theory of operation. Therefore, in this article, we will build a carburetor. Let's go!
An automobile engine is nothing more than an air pump. Because it can create compression when the valves are closed, it can also create a vacuum when the piston goes down and intake valve is open. As the engine cranks, the moving stream of air comes in through an intake-manifold, which runs from each cylinder to the top of the engine. We will use this air stream to make the carburetor work.
Air Horn, Float Bowl & Vent
First, we need a plain round metal tube, which we'll call the air horn. We next attach a bowl to the horn to hold a supply of gas. Inside the bowl, we must provide a float (like that in a toilet bowl). That float will control a needle valve, so that when the bowl fills the upward movement of the float will shut off the flow of gas. The float bowl must be vented to the atmosphere, so gas will flow out when pressure builds up because an unvented bowl, when hot, would cause starting problems.
Next, we need to connect the bowl to the air horn with a small tube, called the discharge tube, and the nozzle at the end of the tube must be positioned higher than the level of gas in the bowl. No gas will flow out unless we create a vacuum in the air horn. By building a narrowing (restriction) in the air horn, the moving air will be sped up, creating an additional, localized, vacuum. In physics, this is called the "venturi principle". This narrowing in the carburetor is therefore called the Venturi. Many modern carburetors utilize a venturi-within-a-venturi to further speed up air flow and help atomize the gas. The discharge tube is placed in this "secondary" venturi in our drawing.
Our Tube now has a venturi, along with a discharge tube.
At this point in our construction, gasoline would be drawn into the tube and out the nozzle, but the drops would be somewhat large. Since we need to make the drops as small as possible — for atomization — we need to add air to the fuel as it moves through the nozzle. A small tube — called an "air-bleed" — is attached to the main discharge tube for this purpose.
Addition of Air Bleed causes dropplets of fuel to be much smaller.
Still, our engine won't run properly because we haven't done anything about maintaining the proper air-to-fuel ratio (remember?). This is easy to fix, however, since all we need to do is to provide a metering orifice — "jet" — in the discharge tube. The jet's orifice size is calculated according to the engine's internal dynamics by the engineers who designed the engine. With this jet in place, the engine will be able to run at a constant speed of 2500 or more RPM.
Main discharge jet controls amount of fuel entering the discharge tube.
Unfortunately, at this point in the construction of our carburetor the engine will not start! When cold, the engine needs a mixture rich with gasoline, so that enough vapor will be produced to get it started. The solution is simple, since we only need to shut off some of the engine's air supply. If we put a plate over the top of the air horn, vacuum from the intake stroke will pull more gas from the discharge tube, providing a proper starting mixture. This plate is called a "choke", and it can be operated manually or automatically. Our engine will now start, but still won't run at anything other than wide-open, because we haven't provided any way of modulating its speed. Not to worry!