The main carburettor parts play a critical role in the workings of an internal combustion engine. The carburettor’s function is to mix fuel and air and then send it to the engine’s cylinders. These cylinders ignite the mixture using spark plugs or an equivalent, which is what powers the engine.
Virtually all matter relies on the presence of oxygen to create fire, be that paper, electrical wiring or, in this case, fuel. The fuel inside an engine can’t ignite if oxygen isn’t present. It’s the interaction of oxygen with fuel that allows the cylinders to create the combustion cycles that power the engine.
In short, without a carburettor, an internal combustion engine simply won’t run.
We usually associate carburettors with motor vehicles, but various types of outdoor power equipment also need them, especially those used in the garden. The most obvious one is a lawnmower, but leaf blowers, lawn trimmers and power tools like chainsaws also have carburettors.
JAK Max specialises in supplying wholesale carburettor parts for the outdoor power equipment industry. Only 20 minutes from the Melbourne CBD, we have 14 years of experience, industry knowledge and networking with the world’s top suppliers.
Here’s our guide to how all the various parts of a carburettor function to keep an engine running.
List of the main carburettor parts
Below is a cross-section of a basic carburettor, with its primary parts highlighted.
Carburettor parts diagram
Let’s discuss each part in turn.
1. Venturi Tube
As you can see from the diagram above, the carburettor venturi tube is a narrow point in the vertical carburettor pipe. It serves to speed up the flow of the fuel and oxygen mixture. As it passes through the constriction, the fuel is forced to move faster. This also reduces the pressure in the area, creating a partial vacuum.
As nature abhors a vacuum and has to fill it, it does so by sucking in fuel from the carburettor’s float chamber and air from its air chamber at the top. The venturi tube is one of the carburettor parts that plays a central role in ensuring that the cylinders receive the right balance of fuel and oxygen for optimal combustion and engine performance.
2. Float Chamber
A float chamber functions to automate and regulate the supply of fluid in an engine. Thus a carburettor float chamber measures and regulates the amount of fuel going to the engine’s cylinders.
All float chambers work on the same principles. In fact there’s even one in your toilet’s cistern, so you can watch it in action for yourself. The float chamber contains a hollow object floating on top of the fuel. (That’s the big ball in your cistern.) This is linked by an arm to an inlet valve that controls the fuel intake.
When the float chamber is filled, the fuel makes the float move upwards, causing the float arm to close the valve. As the level of fuel drops, so does the float, and the arm opens the valve to allow fuel to flow into the float chamber. These carburettor parts enable the float chamber to keep a consistent fuel level and pressure, and create a steady rate of fuel flow into the carburettor.
3. Jets
Carburettor jets are small brass fittings positioned between the float chamber and the venturi tube, as seen in the diagram above. Without getting too technical, they work on Bernoulli’s principle of fluid dynamics.
To you or me, this means that the more air flowing through the venturi tube (remember that vacuum effect?), the more fuel is sucked through the jet into the vertical carburettor housing. This is yet another factor in the overall fuel flow control system.
There are four main types of carburettor jets:
- Pilot jet – this is the jet that manages the amount of fuel that flows into the venturi tunnel when the engine is idling
- Main jet – this jet opens to control the fuel flow when you open the engine’s throttle to 50% – 100% power
- Jet needle – this controls the flow of fuel when you both open and close the throttle. It functions within a range of 20% – 80% throttle power.
- Needle jet – the jet needle enters and exits this jet as it regulates fuel flow.
The main jet also functions to regulate the fuel-air mixture and ensure that the correct air-to-fuel ratio reaches the engine. The size of the jet can affect this, depending on the circumstances.
A small jet restricts the fuel flow, resulting in the fuel-air ratio containing more oxygen. This can increase fuel economy, but at the expense of engine power.
A larger jet allows more fuel to enter the venturi tunnel, which increases engine power, but decreases fuel economy.
4. Throttle Valve
The carburettor throttle valve is also called a piston valve. It’s one of the most important parts of a carburettor, as it regulates the air flowing into it.
This is a butterfly valve that works alongside the main jet, in response to accelerator pedal or engine throttle changes. The main jet will open when the throttle valve opens in response to increased pressure on the throttle. This allows the venturi tube to pull fuel from the main jet at the right rate compared to the airflow, to maintain a steady fuel-air ratio.
The throttle valve is linked to the engine’s accelerator pedal by a spring-loaded arm. It’s a simple direct mechanism that causes the arm to open the butterfly valve when the accelerator is depressed. A smooth and precise throttle response is crucial to ensure that the optimal fuel-air mixture is instantly available to the engine, so it can perform the required task optimally. This also helps with engine durability.
5. Choke
The carburettor choke is also a butterfly valve. Its role is to reduce the amount of air into the carburettor so that the fuel-air ratio is richer in fuel. This allows a cold engine to be started more easily.
The effect of the choke restricting airflow is that this lowers the intake manifold pressure, causing more fuel to enter the carburettor. It’s the partial vacuum effect in action again.
Engines can either have a manual or an automatic choke. The difference between them is exactly what you’d expect. A manual choke has a lever that the user pulls to activate it, while an automatic choke is connected to the accelerator by a metal spring. This opens a choke plate when the accelerator is fully depressed before starting the engine.
6. Idle System
The carburettor idle system is responsible for monitoring the fuel supply when the engine is idling. This is necessary because when the engine is running at low RPM, the throttle is almost closed. This lowers the speed of the air flowing into the carburettor, which can interfere with the air-fuel mix and cause stalling.
The idle system is basically the whole passage from the float chamber to the venturi tube in the carburettor. It enriches the fuel mixture when the engine is idling to keep the engine running smoothly.
You can control the idle mixture using the idle mixture screw, typically located on the side of the carburettor. You can adjust the mixture and the idle speed control by turning the idle mixture screw counter-clockwise – you won’t need more than one to one-and-a-half turns.
Improper idling can not only interfere with current performance, but can also lead to long-term engine degradation.
7. Linkages and Springs
Carburettor linkages are essentially the mechanical arms of the system that open and close valves, for instance. Carburettor springs are used in conjunction with the linkages to enable certain automated movements and ensure that they maintain their proper positions.
A carburettor uses a variety of differently shaped and angled linkages to connect its components and control its systems. For example, the aforementioned linkage that allows the float in the float chamber to open and close the fuel intake valve.
Springs reinforce the linkages to ensure that the carburettor components stay in the correct position. They also allow the arms to snap back quickly from extended positions, in which case they are known as return springs. Similarly, springs help the carburettor’s valves to return to and maintain their closed positions when required to balance the air-fuel ratio.
This combination of linkages and springs allows for higher component stability and contributes towards a smoother throttle response by acting as movement regulators.
8. Gaskets and Seals
There are various types of carburettor seals, all with the same function – to do precisely what their name says. For example, dirt plug seals stop any undesired material from entering the carburettor. Throttle valve seals prevent air from leaking into the system, disturbing the air-fuel ratio.
Carburettor gaskets are a type of seal that specifically prevent air or fuel leakage between the carburettor and the engine.
These are two small but vital carburettor parts that ensure the integrity of the entire system. They’re hugely important for optimal engine performance.
9. Fuel Inlet and Fuel Filter
The carburettor fuel inlet is the entrance to the carburettor to which the fuel inlet lines attach, thus feeding fuel to the carburettor. The fuel goes directly into the float chamber, and the fuel inlet regulates the flow rate.
A fuel filter prevents debris from entering the carburettor by removing foreign objects like rust particles or dust from the fuel. It’s one of the most critical elements of the system, not only to protect the carburettor from wear and tear, but also to ensure clean fuel. This is essential for reliable engine operation.
10. Mounting and Air Cleaner
The carburettor is mounted on top of the engine. In a typical configuration it sits on the cylinder head, between the intake manifold and the air cleaner.
The carburettor air cleaner is the large round disc with ruffled edges that you’ll usually notice first when opening a car bonnet. Its role is to filter impurities from the air before it enters the carburettor, such as dust. This is essential for maintaining the carburettor’s efficiency.
Proper mounting is important to establish the carburettor’s correct place in the fuel and air supply chain, to ensure that the right air-fuel mixture reaches the cylinders.
Best brands that manufacture carburettor parts
In Australia you have a wide choice of brands that produce parts for carburettors. Here’s a list of those you’ll find here at JAK Max:
If you’re wondering where to buy carburettor parts, you don’t need to look any further than our online catalogue for a great selection.