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!
Choke: A. Choke Valve open, air passing through airhorn. B. Choke Valve closed. Vacuum from intake drawing on discharge nozzle.
If we put a plate in the bottom of the tube — under the venturi and above its mounting to the engine — pivot it from its centerline and connect the proper linkage to it, we now can control the amount of air/fuel mixture reaching the cylinders at any given time. This is our throttle valve, commonly known as the throttle or accelerator. At this stage, our basic carburetor is still not complete. We can't idle it without stalling; it will have little power at speeds just above idle; and whenever the throttle is quickly opened there will be a "flat-spot" until the engine can run up in speed.
Throttle Valve regulates flow of fuel mixture. Shown in wide open, half open and closed positions.
Back to work. It should be clear by now that a proper carburetor must contain a number of separate fuel-system devices. The float, choke and throttle are three of them, but we still need others to provide the necessary air/fuel ratios to run the engine under other conditions. Let's categorize them:
1. Idle. A ratio of 12:1 is common for proper idling.
2. Low speed. A ratio of 16:1 is necessary for part-throttle (30-65 mph) operation.
3. High speed. A ratio of 13:1 is necessary for full-throttle operation.
4. Full acceleration: A ratio of 14:1 is needed.
5. Starting cold. A ratio of 8:1 is needed.
We have taken care of the engine's needs for starting and full-throttle operation. Now, let's create some circuits to solve the other problems.
Idle circuit: If we create an additional passage off the main discharge tube and run it down below the throttle plate and out an orifice in the air horn, the engine's vacuum will draw in fuel for idling. Normally, carburetors are built with an adjusting valve so that the amount of fuel can be varied to produce the best idle, commonly called the "idle mixture" screw(s). Without such an adjustment an engine would run too rich at idle, since what's happening is that fuel is dripping into the engine through a process of "controlled leaking."
Now we need to make the engine run smoothly at part-throttle. As soon as the throttle is opened past the idle position, more fuel mixture is necessary. However, there still isn't enough airflow through the venturi to cause fuel to be drawn through the main discharge nozzle. If we utilize that passage we developed for the idle circuit and drill a few orifices just above the throttle plate's closed position, additional fuel will be drawn out of these as the plate is opened. As each hole is uncovered, more fuel flows, providing power until the main discharge nozzle functions. Things are getting better, but —
Our carburetor now has an idle circuit, and when the throttle is partly open, additional fuel is drawn in through the Low Speed Orifice.
We still have one additional problem and that's a "flat-spot" under hard acceleration. This is due to a momentary lack of vacuum when the throttle plate is suddenly thrown open. To compensate for this, an acceleration pump circuit has been designed in most carburetors. This circuit is usually operated by linkage to a pump chamber in the carburetor. When the accelerator is pushed down, fuel is sprayed into the air horn or the venturi. Another, non-linkage type of acceleration circuit is the power jet circuit. This system utilizes a vacuum-held piston which — when vacuum decreases — is pushed down by a spring, thereby pumping fuel.
At last, we have a carburetor that will run an engine very well, but only a relatively small one. What we have shown here is a one-venturi (one-barrel) carburetor. As engines became larger manufacturers modified the carburetion systems to better distribute fuel to multiple cylinders, thus producing more power. The day of the one-barrel carburetor was pretty much over by the early 1960s.
Many cars use two-barrel and four-barrel carburetors and some others use multiple carburetors (two four-barrels, three two-barrels, etc.) Multiple-barrel carburetors are just the same as single ones. They simply utilize common float bowls, chokes and other items in one housing for efficiency. Rebuilding any of them is not mysterious. All you need to remember is to recognize each circuit in your carburetor and don't forget any parts! All the external stuff is there for such things as fast-idle, choke actuation, air conditioner speed-up, vacuum takeoff and mixture preheat.
Take a little extra time to study your car's manual to familiarize yourself with everything and then go to it. There's nothing to be afraid of.
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