Understanding How Carburettors Work

Whilst most modern cars now use fancy fuel injection systems, for fans and owners of classic cars, it is the carburettor that we must tame if we desire trouble-free motoring. 

We've taken advice from Calan at Cartoon Motors, who has kindly written this detailed guide on how they work, how to adjust them, and how to keep them running smoothly.

Following on from previous technical articles including topics around engine oil, brake fluid, and campervan electrics let us shine a spotlight on the carburettor. 

Well, the easiest way to put it is this: A carburettor or carb is the old mechanical version of a fuel injector or throttle body. It is the component of an internal combustion engine that mixes air with a fine spray of liquid fuel, typically gasoline, to create a combustible mixture for your motor. It also regulates the ratio of air and fuel to optimize engine performance.

Carburettors are no longer used in today’s cars due to the superior efficiency of fuel injection systems. However, many older classics will have them fitted, and they can still perform wonderfully when looked after. Plus, being mechanical, there's a chance you can fix them if they break! Shop for carburettor parts here. 

A carburettor works by utilising the Venturi effect.

This effect comes about because of a narrowing passage in the carburettor (commonly known as the barrel) that increases the airflow velocity, creating a low-pressure area. This pressure drop creates suction and draws fuel out of a reservoir (float chamber) through jets or nozzles and atomizes it into fine droplets.

Afterwards, the atomized fuel mixes with the incoming air in the barrel, resulting in an air-fuel mixture with the appropriate ratio for combustion. This mixture is then delivered to the engine’s cylinders, where it can be compressed and ignited to produce power to propel your car.

A carburettor consists of many different parts working together, often making them look complicated. However, as we break it down and explore the main components one by one, it rapidly becomes apparent that a carburettor is neither complicated nor magical. It’s all just mechanical logic.

The Carburettor Barrel

The barrel, Venturi tube, or throat amongst many other names, refers to the constricted passage where the airflow speeds up, creating a low-pressure area. This design allows the carburettor to draw in and mix the right amount of fuel with the incoming air, ensuring efficient combustion in the engine.

This passage connects the air filter to the intake manifold and must be properly designed to generate enough suction for optimal carburettor performance.

The Throttle Valve

A carburettor will usually contain two different butterfly valves per barrel. The lower one is your throttle or intake valve. This valve controls the amount of air entering your engine and is directly connected to your accelerator pedal. It can either be connected with an accelerator cable or in more modern cars, an electrical system which opens and closes the valve instead; this is referred to as  'drive by wire'. 

By opening and closing the throttle valve, you can control the airflow into the carburettor and subsequently the engine’s power output.

If your throttle valve is operated by a cable, it is crucial to ensure proper cable maintenance for optimal performance.

The Choke Valve

The Choke valve is the first valve you see when you remove the air filter and look into the top of your carb assembly. This valve is pretty straightforward, it chokes the airflow to the engine during cold starts. This restriction creates a richer air-fuel mixture, which is easier to ignite in colder conditions. As the engine warms up, the choke valve gradually opens to allow more air into the carburettor, helping the engine run smoothly at normal operating temperatures. 

This valve can either be manually or electronically controlled.

The Bowl Or Float Chamber

The bowl or float chamber in a carburettor is a reservoir that holds liquid fuel, typically gasoline. A floating mechanism inside the chamber controls the fuel level, ensuring that it remains the same at all times. There must always be fuel in this chamber since the pressure drop caused by the venturi effect draws it out from this small reservoir.

The Jets

The jets in a carburettor, including the main jet and idle jet, are small screws with a hole in the middle of them. They control the flow of fuel into the mixing chamber. The main jet is responsible for delivering fuel during high-speed or full-throttle operation. Whereas, the idle jet provides a controlled amount of fuel when the engine is at idle or in low-speed conditions.

Smaller jets result in a leaner mixture with less fuel, while larger jets provide a richer mixture with more fuel.

The Accelerator Pump

The accelerator pump in a carburettor is a mechanism that provides an extra burst of fuel when the accelerator pedal is pressed suddenly. This quick injection of fuel helps prevent hesitation or stumbling in the engine’s response to sudden throttle changes, improving acceleration and drivability. It ensures that there is an adequate supply of fuel for immediate power delivery when needed.

Carburettors have the distinct benefit of being mechanically simple, allowing anyone to successfully tune and adjust them without sophisticated equipment, provided they know what they are doing!

Surprisingly, the majority of the adjustments can be made using a flathead screwdriver. This hands-on approach often makes carburettors appealing to both enthusiasts and mechanics and it's not uncommon to find classic cars that have had fuel injection systems deliberately removed in favour of a more simple carburettor set-up.  

Idle Speed Adjustment

The Idle Speed adjustment on a carburettor allows you to set the engine’s speed when it’s not under load (at idle). To adjust it, you typically need to locate the idle speed screw on the carburettor and turn it clockwise to increase the idle speed or counterclockwise to decrease it.

A good rule of thumb, a warm engine should idle between 600 to 1000 RPM. Don't try and adjust it whilst it is still on choke. 

This tuning ensures that the engine runs smoothly at idle and prevents stalling or rough idling.

Idle Mixture Adjustment

The Idle Mixture adjustment on a carburettor controls the ratio of air to fuel when the engine is idling. Once again to adjust it, you usually have to locate the idle mixture screw, which can be turned clockwise to enrich the mixture (more fuel) or counterclockwise to lean it out (more air).

The goal is to find the optimal balance that results in a smooth idle while maintaining fuel efficiency and proper air-to-fuel balance.

High-Speed Mixture Adjustment

The High-Speed Mixture adjustment on a carburettor is similar to the idle mixture adjustment. However, this screw controls the air-fuel ratio at higher engine speeds instead of at idle. Typically during full-throttle, acceleration or high-speed operation.

Just like the previous adjustment, you locate the high-speed mixture screw and turn it to fine-tune the mixture. 

Choke Adjustment

The Choke adjustment on a carburettor controls the operation of the choke valve, which restricts airflow during cold engine starts to provide a richer air-fuel mixture. Typically to adjust the choke, you need to locate the choke adjustment mechanism and set it according to the manufacturer’s recommendations.

The adjustment mechanism can vary from one carburettor to another. Proper adjustment ensures that the engine starts smoothly and quickly in cold weather.

Float Level Adjustment

The Float Level adjustment on a carburettor sets the height of the float inside the float chamber, which controls the amount of fuel that can enter this chamber. It is not common for the float to require adjustment. Typically speaking, once it is set it shouldn’t move or de-adjust itself.

However, if needed, you typically have to measure the distance between the float and the carburettor body while the carburettor is inverted. From there, adjustments are made by bending the float arm or using adjustment screws if provided.

Accelerator Pump Adjustment

The Accelerator Pump adjustment on a carburettor allows you to fine-tune the amount of additional fuel sprayed into the engine when you press the accelerator pedal suddenly. Just like the majority of the other adjustments, there is commonly an adjustment screw or mechanism associated with the accelerator pump that makes clockwise adjustments for a richer mixture (more fuel) and counterclockwise for a leaner mixture (more air) or vice versa.

Main And Idle Jet Adjustment

The Main and Idle Jet adjustment on a carburettor involves fine-tuning the flow of fuel through these jets, which in turn controls the air-fuel mixture. To make this adjustment, you have to locate these jets in the bowl and replace them with different-sized jets. Smaller jets lead to a leaner mixture (more air), while larger jets create a richer mixture (more fuel).

Proper car maintenance is a crucial part of classic car ownership and carburettors are one of the things that need to be included in your routine maintenance.

Carburettor maintenance can consist of a good visual inspection for signs of wear, loose bolts, split seals or gaskets and damaged components, and cleaning and even rebuilding if required.

The interval at which you should perform maintenance will vary greatly on how often you drive your car and the way that it behaves. If your vehicle is running well, then just conduct a visual check, rather than risk removal for the sake of it. You don't want to encourage something to go wrong!

Should you feel your carburettor isn't functioning correctly, removal and rebuild can be a sensible option. Alternatively, a new carburettor could be a more cost-effective choice, especially if you are paying for professional assistance. 

If you are going down the rebuild route, source a carburettor gasket set for putting it all back together again first... You are unlikely to be able to reuse these.

To perform this process remove the carburettor from the car, being careful not to let any dirt fall into the manifold. If the carb is particularly dirty you may choose to clean it in situ with carburettor cleaner and a soft brush, first, before removing cables, fuel and vacuum pipes and any electrical connections. 

With the carburettor on a clean workbench, you can methodically strip it down, take photos or use a notepad to ensure you can put it all back together again. 

Soak all disassembled parts in a carburettor cleaner solution to remove built-up deposits and varnish from old fuel. You can also use compressed air or a thin metal wire to remove any remaining debris from the small passages and jets. 

Once cleaned, reassemble the carburettor, ensuring gaskets and seals are new or in good condition, and secure all components snugly. Finally, reinstall the air filter housing, start the engine, and fine-tune the carburettor adjustments as per your workshop manual.

So, whether you’re a classic car owner or simply curious about automotive engineering from the past, we hope our exploration into carburettors has shed light on a vital component below the bonnets and deck lids of the older vehicles you know and love. 

Calan, Cartoon Motors