How A Carburettor Works

The first thing to ask; what is carburettor?

In very simple terms; a carburettor is a device found in older engines, muscle cars or drag racing cars. Its sole purpose is to blend air with a fine mist of liquid fuel, creating a ready mixed product ready for combustion by the cylinders in the engine. Think of it as the engine’s quality assurance engineer that is carefully preparing the right blend of air and fuel to power the vehicle depending on the requirements of the engine.

To understand how to install or adjust a Holley ‘Classic’ series carb I have the Holley PDF file available to download below:

Back to how a carb works:

The carb is a highly complex piece of equipment often misunderstood and overlooked. To give you an idea of what is inside a Holley 4160 series carb fitted on Fords between 1964 – 19676 the exploded diagram below gives you a visual of the parts involved.

Just one of the components can cause issues with running, starting, idling, acceleration etc. Hopefully a few of the bigger carb part and mentioned definitions below will clarifying things a little bit.

1. Air Intake: The carburettor sits atop the engine block, beneath the air filter. It draws in fresh air from the surroundings.
2. Fuel Mixing: Inside the carburettor, fuel is mixed with the incoming air. The goal is to achieve the right air-fuel ratio for efficient combustion. Too much fuel or too little air can lead to poor performance or excessive emissions.
3. Venturi Effect: The carburettor uses a venturi—a narrow passage where the air speeds up due to constriction. As air flows through the venturi, it creates a low-pressure area, which draws fuel from the carburettor bowl.
4. Throttle Valve: The throttle valve controls the amount of air entering the engine. When you press the accelerator (gas) pedal, the throttle opens, allowing more air (and fuel) into the engine.
5. Float Chamber: The carburettor has a float chamber (also called a float bowl) that holds a reservoir of fuel. A float-controlled needle valve regulates the fuel level. As the fuel level drops, the float opens the valve to allow more fuel back in.
6. Choke: The choke restricts the air supply during cold starts. It enriches the air-fuel mixture by reducing the airflow, making it easier to ignite the engine.
7. Atomization: The carburettor atomizes the fuel, breaking it into tiny droplets for better combustion. These droplets mix with the incoming air.
8. Combustion: The air-fuel mixture enters the engine cylinders, where it’s ignited by a spark plug. The resulting combustion (along with ignition trimmings) process generates power on each power down stroke of the cylinder (four stroke) to drive the vehicle.

Not all carbs are the same obviously as they have different adjustments, fittings, mountings and specific applications. However the principle is the same, the fuel and air need to be mixed at idle, normal driving and heavy load or acceleration. For basic or smaller sized engine cars there are single valve designs, two barrel or twin valves perform the mixing of the fuel and air for the engine to work. Mostly these style of carbs either single or two barrels are common in standard older everyday or more fuel efficient road cars. What is the difference between the two and four barrels and do we really need another two barrels?

The primary differences between a Holley two barrel and a four barrel carburettor is their design, functionality and application, which will affect the air-fuel mixture delivery to the engine making it more efficient and delivering more fuel and air mix, in turn more power and or torque.

Holley Two Barrel Carburettor

1. Number of Barrels: As the name suggests, a two barrel carburettor has two venturi or barrels through which air and fuel are mixed and delivered to the engine.
2. Air-Fuel Flow: It has a simpler design with fewer moving parts and typically provides a lower volume of air and fuel mixture. This makes it suitable for engines that do not require high levels of fuel flow, like those in everyday street vehicles.
3. Fuel Efficiency: Generally, two barrel carburettors are more fuel-efficient, as they deliver a steady and controlled amount of fuel suitable for normal driving conditions without excessive fuel consumption.
4. Performance: They offer adequate performance for smaller or less powerful engines and are not designed for high-performance applications. They are often found in standard passenger vehicles, where fuel efficiency and drivability are prioritized.

Holley Four Barrel Carburettor

1. Number of Barrels: A four barrel carburettor has four venturi, essentially doubling the potential air and fuel flow compared to a two barrel carburettor. This configuration includes two primary and two secondary barrels.
2. Air-Fuel Flow: This type of carburettor can provide significantly more air and fuel to the engine, which is beneficial for high-performance applications where more power is required, such as in racing or muscle cars.
3. Fuel Efficiency and Performance: While they can deliver more fuel, four barrel carburettors can also be more fuel-efficient under normal driving conditions because the secondary barrels only open when additional power is needed. This allows the engine to run on the primary barrels during light load conditions, conserving fuel.
4. Application: Four barrel carburettors are designed for performance-oriented vehicles that require greater fuel flow to achieve higher power outputs. They are more complex, with additional components to manage the secondary barrels’ opening.

In summary, a single or two barrel carburettor is typically used in standard vehicles for everyday use, offering simplicity and fuel efficiency. In contrast, a four barrel carburettor is used in high-performance applications, providing the capability for greater power output and higher fuel flow when needed.

Drag racing cars use high CFM (Cubic Feet per Minute) carbs by default as they deliver more power due to the higher amounts of fuel that can be produced and mixed with more air. Various configurations of carbs on spacers or types of opening in the manifold will produce more low down torque or higher top speed. Drag racing crews often state that carbs use less fuel than EFI (Electronic Fuel Injection). During drag racing the carb valves are held wide open to give the maximum intake of fuel and air for combustion to give power and torque, the racing crews are not particularly worried about idle or general driving. The carb can also be paired with super chargers to compress even more fuel air mix into the engine giving even more power. That is whole nother story for another day.

Carb manufacturers often use similar terminology such as these:

Primary Valves

1. Function: The primary valves are the main components for regular engine operation, handling most of the air-fuel mixture delivery during idle, cruising, and light acceleration.
2. Components: They include throttle plates (butterflies) that open as the driver presses the accelerator, allowing more air to enter the engine. The fuel is mixed with this air in the venturi, a narrow section that increases the velocity of the air and helps atomize the fuel.
3. Operation: These valves open gradually in response to the throttle position, providing a smooth and controlled fuel delivery for efficient engine performance.
4. When a carb fitted car starts up they are usually running a ‘choke’ to give even more fuel (rich) mixture while the car warms up. This process is normally a mechanical or electrical plate that closes or partially closes the air intake on the primary valves

Secondary Valves

1. Function: Secondary valves provide additional air-fuel mixture during heavy acceleration or when extra power is needed, such as during rapid acceleration or high-speed driving.
2. Components: Like the primary side, the secondary side also has throttle plates, but these are typically larger to allow a greater volume of air and fuel mixture when open. Some carburettors use mechanical linkage, while others use a vacuum mechanism to control the opening of the secondary valves.
3. Operation: The secondary valves open either mechanically (linked to the primary throttle plates via a cam) or via a vacuum system that senses engine load and demand for additional air and fuel. They remain closed during normal driving to conserve fuel.
4. Secondary cams can be custom adjusted to come on earlier or later as required for the type of power delivery required.

Differences of primary and secondary valves

1. Size and Flow: Secondary valves are usually larger than primary valves, allowing for increased air and fuel flow during high-demand situations.
2. Control Mechanism: The primary valves are directly controlled by the accelerator pedal, while the secondary valves may use a mechanical linkage or vacuum actuation to open. This setup allows for a more progressive and efficient increase in power.
3. Fuel Delivery: The primary circuit handles most of the fuel delivery, ensuring efficient operation under normal conditions, while the secondary circuit kicks in for performance situations, providing an extra boost of power.
4. Under snap or sharp acceleration there are additional jets that squirt the primary and secondary valves to boost the initial fuel mixture. If this wasn’t the case when the valves open, the initial suck in of air would run to lean and not give enough power to the cylinders.

Manufacturers of Carbs

Holley and Edelbrock are two of the most popular brands of carburettors, especially in the aftermarket performance market. While both companies produce high-quality carburettors, there are distinct differences in their design, tuning, and performance characteristics. I will concentrate on these two manufacturers for now. There are many other specialist manufacturers of carbs for specific types of applications.

Holley Carburettors

Design and Features:

• Modular Design: Holley carburettors are known for their modular design, which allows for extensive customization. They typically feature replaceable jets, power valves, and accelerator pumps, making them highly tunable.
• Mechanical or Vacuum Secondaries: Holley offers models with both mechanical secondaries, which are linked directly to the throttle, and vacuum secondaries, which open based on engine demand.
• Double Pumper Options: Holley’s double pumper models use dual accelerator pumps, providing an extra fuel shot for aggressive throttle applications.

Pros:

1. Customization: Highly customizable due to the wide range of interchangeable parts, making them suitable for fine-tuning specific performance needs.
2. Performance Focused: Holley carburettors are often favoured in racing and high-performance applications due to their tunability and ability to support significant horsepower.
3. Availability of Parts: Extensive availability of aftermarket parts and tuning accessories.

Cons:

1. Complexity: The high level of tunability can make them more complex to set up and adjust, which can be daunting for beginners.
2. Maintenance: Requires more frequent maintenance and tuning to keep them running optimally, especially in high-performance environments.

Edelbrock Carburettors

Design and Features:

• Simple Design: Edelbrock carburettors feature a simpler design with fewer parts compared to Holley, which can be advantageous for ease of use and reliability.
• Mechanical Secondaries: Typically, Edelbrock carburettors use mechanical secondaries that are linked to the throttle position, providing a more straightforward operation.
• Electric Choke: Many Edelbrock models come with an electric choke, which simplifies cold starts and improves drivability in various conditions.

Pros:

1. Ease of Use: Generally easier to install and tune, making them more user-friendly, especially for less experienced users.
2. Consistency: Known for their reliability and consistency in performance, with less frequent tuning needed.
3. Fuel Efficiency: Often more fuel-efficient than Holley carburettors, making them a good choice for street-driven vehicles.

Cons:

1. Limited Customization: Less tunable than Holley carburettors, which might limit their performance potential in highly customized or extreme performance applications.
2. Parts Availability: While parts are available, there is a less extensive aftermarket for tuning components compared to Holley.

Summary

• Holley carburettors are ideal for those seeking maximum performance and customization. They are well-suited for racing and high-performance applications but require more knowledge and maintenance.
• Edelbrock carburettors are user-friendly and reliable, making them a good choice for enthusiasts looking for a straightforward installation and consistent performance, especially in daily drivers and lightly modified vehicles.
• There is are parameters to the carb sizing. A to big a CFM will cause ‘bore wash’ where fuel is unburnt in the cylinders and washes the oil lubrications for the cylinders walls. This will also be wasteful of fuel as it’s just being unburnt. This could also cause the engine to flood and some very poor running.
• If the carb is to small not enough fuel air mix is provided and will cause poor running even running hot.

I have found some videos of a carb working under loads. These videos show, start up, revving, initial fuel shot under revving, and full throttle. The second video has a whining sound in the background, this is probably due to a super charger. Regardless of the make, the principles of how they work are exactly the same.

Carb Calculations

To fit a carb you can replace the like for like of course, but to get more power you don’t just slap the biggest carb on the engine you can. To big a carb will cause very poor running and give you the exact opposite you wanted and make the car worse. To small a carb will starve the engine of the full power potential. There are various online calculators to work out the optimal CFM carb size you need for a particular engine size, speed and efficiency.

That formula can seem complicated for calculating the correct CFM for carb on a particular engine. Broken down with an example looks like this:

CFM (optimal) = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456.

Any ordinary stock engine will have a volumetric efficiency of about 80%. Most rebuilt street engines with the usual average bolt-ons have a volumetric efficiency of about 85%, while race engines can range from 95% up to 110% efficiency.

Example: Using a 355 CID engine x 5,500 max rpm = 1,952,500

Take 1,952,500 x .85 = 1,659,625

Then 1,659,625 ÷ 3456 = 480 CFM

Even with about a 10% cushion, a 500 CFM carburettor will handle this engine great. If you try this formula yourself, be honest with how much rpm the engine will see. You’d be better off with a carb smaller than needed rather than something oversized that can lead to poor drivability and performance.

Higher Tuning

There are various ways to improve, or fine tune a carb to the exact requirements. This could be changing of ‘jets’ or ‘cams’ as required.

1. Pilot Jet: Located in the float bowl, the pilot jet provides fuel for starting, idling, and the initial 20% of throttle range. It draws fuel through small holes as air flows through the carburettor.
2. Main Jet: Larger and usually screwed into the needle jet, the main jet controls the air/fuel mixture from around 75% throttle to full open throttle (100%).
3. Needle Jet: This critical component controls fuel flow between 20% and 75% throttle. It consists of the hollow needle jet and the jet needle, which adjusts fuel flow as you open or close the throttle.
4. Air Screw: A small brass adjustment screw near the carburettor inlet, the air screw works with the pilot jet for starting, idling, and initial power delivery (up to 20%). It allows slight adjustments based on atmospheric conditions.

Cams: are a mechanical parts that will determine when (usually) the secondary valves open and close. The cams can be made to bring on the deployment of valves earlier, later, wider or smaller openings.

Float bowls: will determine when the idle reserves of fuel or secondary valves reserves of fuel, again critical for smooth running.

Spacers Style:

Choosing between a 4-hole versus an open carb spacer ultimately depends on your desired outcome. The table below has a comparison of the features or pros and cons.

Feature	4-Hole Carb Spacer	Open Carb Spacer
Design	Contains four holes arranged in a specific pattern	Does not have any holes or restrictions
Airflow	Provides a more controlled and directed airflow	Allows unrestricted airflow
Fuel Atomization	Enhances fuel atomization by promoting turbulence	May not have as significant an impact on fuel atomization
Low-end Torque	Improves low-end torque by increasing air velocity	May have a lesser impact on low-end torque
Mid-range Power	Provides good mid-range power due to improved mixture formation	May not have as pronounced an effect on mid-range power
Top-end Power	May have a slight reduction in top-end power compared to an open spacer	May provide better top-end power due to unrestricted airflow
Fuel Economy	Can potentially improve fuel economy due to better mixture formation	May not have a significant effect on fuel economy
Engine Response	Enhances throttle response and drivability	May not have as noticeable an impact on engine response
Intake Noise	May reduce intake noise due to the controlled airflow	May increase intake noise due to unrestricted airflow
Application	Often used in street and mild performance applications	Commonly used in high-performance and racing applications

If you prioritize low-end torque and drivability, go for the 4-hole spacer. For high RPM power gains, consider an open spacer. Keep in mind that individual engine setups may vary, so experimentation is key.

Spacer Thickness

Higher carb spacers can offer several benefits, especially in high-performance applications:

1. Increased Plenum Volume: A taller spacer creates a larger plenum volume between the carburettor and intake manifold. This extra space allows for better air distribution and smoother airflow transitions.
2. Improved High-RPM Performance: The increased plenum volume enhances top-end power by allowing more air to accumulate during high RPMs. This can lead to better horsepower and torque at the upper end of the RPM range.
3. Reduced Heat Transfer: A taller spacer can reduce heat transfer from the engine to the carburettor. Cooler air entering the carburettor can improve performance and prevent vapor lock.
4. Enhanced Throttle Response: The additional plenum volume can result in crisper throttle response, especially during rapid acceleration.

Spacer Materials

The choice of spacer materials can make a difference to the running of the car depending on the application of use. There are basically four types of spacer

1. Aluminium Spacers:
   • Advantages:
     • Strength: Aluminium spacers are durable and can withstand engine vibrations.
     • Heat Resistance: They handle high temperatures well.
     • Lightweight: Aluminium spacers add minimal weight.
   • Disadvantages:
     • Heat Transfer: Aluminium conducts heat, potentially affecting fuel temperature.
     • Cost: They tend to be more expensive.
2. Phenolic Resin Spacers:
   • Advantages:
     • Insulation: Phenolic spacers insulate against heat transfer, keeping the fuel cooler.
     • Affordability: They are cost-effective.
   • Disadvantages:
     • Not as Strong: Phenolic spacers may not be as robust as aluminium.
     • Limited Heat Resistance: They have extreme high temperature limits.
3. Plastic (Polymer) Spacers:
   • Advantages:
     • Insulation: Like phenolic spacers, plastic spacers insulate against heat.
     • Lightweight: They don’t add significant weight.
     • Affordable: Plastic spacers are budget-friendly.
   • Disadvantages:
     • Durability: They may not be as durable as aluminium.
     • Heat Limitations: Plastic can melt under extreme heat.
4. Wood Spacers:
   • Advantages:
     • Insulation: Wood provides natural insulation against heat transfer.
     • Customizable: Wood spacers can be easily shaped or modified.
     • Affordable: They are usually cost-effective.
   • Disadvantages:
     • Durability: Wood may not be as durable as other materials.
     • Moisture Sensitivity: Wood can swell or warp if exposed to moisture.

Make a decision based on what you need. I picked the four hole design as I prefer low end torque also I don’t thrash my car at top speed – ever. Also a Phenolic spacers helped with fuel evaporation from the carb when hot. One inch spacer gives better torque as well as the four hole design. At lower speeds if I give the pedal some, the car responds straight away.

Tip:

More often than not, when playing with carb settings do little bits at a time to see what happens. Ask yourself why do I need to adjust the carb, is it running poorly because of poor fuel, water in the fuel, dirty air filter, poor plug gaps, points need adjusting? Then adjusting the carb to sort out poor running will also give you poor running when those consumables are replaced or good fuel is put back in. I know that my car runs badly, coughs and splutters on idle if I don’t use premium fuels, Shell V-Power is my default fuel and will drive a few extra miles in order to use their fuel. I never put E10 fuel in my car for that reason.

Don’t change everything in one go as you will be in for a whole heap of hurt chasing your tail finding out what has gone wrong. Adjusting mixtures can have a similar effect of changing jets. Fitting different cams or adding spacers and altering mixtures or timings, you won’t know if one item has had a negative effect or a combination of things if the car still runs badly.

To help with the dreaded ethanol and add lead replacement I use Castrol Valvemaster+

There is just so much you can talk about and to be honest, and it’s a bit of a dying art to tune these carbs up; screwdrivers, spanners, vacuum gauges, experience, feel of the response in the car and probably most importantly – by ear.

There is now an evolution of the carb which is electronically controlled and will provide a modern injected fuel take on the carb, but it’s not fuel injection directly into the cylinders as such. These EFI versions offer even more control over the air and fuel delivery which is much superior than their mechanical counter parts. The EFI units offer better and more consistent starting from both cold and hot starting, better economy, consistent idle and running with far less moving parts. The down side is that you are now into computerised adjustments via a small screen, which needs to be in the car, along with that these carbs need sensors on exhausts to evaluate burnt mixtures which in turn allow for on the fly adjustments to lean or richen up the air fuel mixtures as required. They even train themselves to your driving style. This is all well and good if you want the modern spin and have the money, the technical knowhow to install it and set it up correctly. But, many dinosaurs like myself still prefer the dark art of an old school carb.