Just inherited a 1969 Porsche 912 from my father. I have a general understanding of fuel injection from my days drag racing my nitrous assisted ’86 300ZX but I have no clue how a carburated car works and online articles are beyond me. Need the 5yo explanation because right now, it’s basically dark magic.
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Smarter every day did an video on carburetors on youtube, its probably the best explanation you’re going to get
Smarter Every Day made a really good youtube video on it
The gist of it is the engine draws air in through the carburetor and it flows through a tube that gets more narrow. The narrow tube forces the air to speed up which lowers it’s pressure where you put a nozzle at called a jet that’s connected to a supply of fuel. The lower pressure air causes fuel to be drawn through the nozzle spraying it into a mist. The jet is sized appropriately so the amount of fuel drawn is enough to properly mix with the air to cause complete combustion of the fuel.
The piston moves which draws air through the air filter and into a tube with a narrow portion in the middle. Then the Venturi Effect takes over, when air passes through a narrow opening it moves faster and reduces in pressure. There’s an opening in the tube where the narrow portion is and as the air flows into the narrow portion of the tube the pressure lowers and it pulls in fuel from a reservoir. The air and fuel mix and flow into the intake manifold then the combustion chamber where it’s ignited by a spark plug.
There’s a valve on top of the Venturi tube that can prevent the air from flowing in, that’s called a choke. When the car is cold it needs more fuel to ignite so the choke closes. There’s another valve at the bottom of the tube that allows more air to flow in, that’s called the throttle. Once the engine is warm allowing more air in pulls in more fuel and the process speeds up so the engine goes faster.
The bowl of the carburetor is the reservoir, it always has fuel on hand. That works like a toilet tank. There’s a float inside the reservoir, when the reservoir has fuel the float closes a valve and it stops more fuel from flowing in. As the fuel is used up the float falls and that opens a valve allowing more fuel to flow in.
A carburetor does mechanically what fuel injectors do electronically. Instead of relying on airflow and O2 sensors to determine the right amount of fuel to deliver to the engine, a carburetor relies on principles of fluid mechanics to do the same. Air flowing through the venturi, an air passage with a section that narrows, experiences a pressure drop which pulls fuel through ports (called jets) and causes the fuel to atomize. Jets are sized based on desired performance characteristics. Carburetors are less efficient than fuel injectors, mainly because you can only tune them for a specific set of conditions instead of having a computer continuously adjusting fuel flow based on sensor data.
When the engine turns, each cylinder will suck air in its intake stroke. This sucks air from a manifold. Upstream of the manifold there’s the carburetor. Now we said that the engine is sucking air right? Ok that air is sucked through the carburetor. The carburetor is a Venturi pipe, goes wide-narrow-wide. In the narrow part the air is the fastest and has the least pressure. In that narrow part of the carburetor there is a fuel jet. The depression of the air will suck fuel from that jet. The carburetor has also a valve that regulate the air, it’s the throttle. The more you press the accelerator the more air is let in toward the engine. Basically the acceletator control is working by deciding to choke the engine more or less, full throttle the valve is fully open and the engine can get all the air it can suck, when accelerator is fully released the valve will choke the engine to a point it can just idle at minimum rev.
The magic is that the more air you let in the more suction there is on the fuel jet, this will automatically give the right mixture air/fuel without the need of any electric or electronic control. The carburetor includes a little fuel tank that feeds the fuel jet. The tank is kept always at the perfect level by a float and valve assembly. A fuel pump takes the fuel from your tank to this valve that feeds the carburetor tank.
So the carburetor is a very well made shaped pipe with a fuel jet and an air valve and if tuned properly, it does all you need by using just a mechanical link to the accelerator pedal.
Being completely passive and mechanical, tuning, cleaning and maintenance is essential to keep it working properly. It’s not a computer, can’t self adjust. If it gets out of tune it will perform really bad.
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