CC Tech: The Operation and Tuning of a Holley Carburetor – a Modern Twist on Old Tech

Many of us contributors and commenters lived through the era prior to electronic fuel injection.  This meant that most of us have had the “pleasure” of owning and driving cars with carburetors.  There have been many comments posted over the years speaking of the misery of operating vehicles equipped with carburetors.  To the contrary, the consensus is that EFI is an immense improvement.  I wholeheartedly agree that modern electronic fuel injection was one of the most revolutionary systems added to cars in modern times.  It allowed engines to run far more efficiently.  EFI engines are able to meet ever stricter fuel economy and emission standards without having to sacrifice performance or driveability.  In the dark days of mid-1970s to the early-1980s, tuning engines to meet the emission and fuel economy standards with a carburetor often resulted in serious sacrifices to driveability and performance.

Nevertheless, I am a firm believer that a good running carburetor is wonderful thing and it can deliver excellent all round performance, even in modern times.  The simplicity of a fully mechanical device that can properly mix the fuel and air to feed an engine and that can be overhauled for less than $100 in few hours does appeal to small segment of people.  I am one of those people.  Just in case you think my rose colored glasses prescription is a little too strong, let me provided full disclosure.  I have a strong penchant for mechanical devices and enjoy tinkering.  I have continuously owned a carburetor equipped car for my entire driving history and to this day drive them on a regular basis.  In fact, until 2006, I only owned vehicles equipped with carburetors exclusively.

Performing the maintenance on an original factory carburetor isn’t overly difficult.  It mostly involves adjustments and the occasion tear-down and overhaul. A factory carburetor was calibrated to operate on a specific engine by some highly skilled engineers and they don’t really need major changes to their calibration or operation.  Keep up with the maintenance and they usually will run just fine.

Ford’s Variable Venturi Carburetor was not one of Ford’s better ideas. Owners often replaced these with another carburetor such as Motorcraft 2100 or a Holley 2bbl.


That said, there were lots of carburetors that were not well calibrated and ran poorly even when new.  Beginning in the early 1970s, carburetor calibrations were seriously compromised to meet emission standards.   Those carburetors from the mid-1970s just before catalytic converters were particularly poor.  They were calibrated overly lean and had many driveabilty issues from stalling when cold to poor performance.   There were many cars that came equipped with these awful or cantankerous carburetors.  There are also some carburetors that have little to no support today.  Then there are those owners that modify their engines to improve performance which often requires a more high performance oriented carburetor.  The simple solution to all these problems is to replace the carburetor.  If the engine in question is a traditional American V8, then there are many new aftermarket carburetors available

The aftermarket offers many throttle body EFI systems that can easily be retrofitted in place of a traditional carburetor. More expensive multi point systems are also available for some applications.

Despite the fact that simple to install throttle body EFI kits are readily available, the number of carburetors available today has seemingly increased.  Reading through the carburetor descriptions and using tech support one can usually narrow the choice down based on the displacement, RPM operating range, camshaft, and approximate horsepower.  If you believe the ad copy though, you should be able to bolt this new carburetor on your engine and with a quick couple of adjustments be ready to run.  The fact is, that carburetor might be in the ball park to run well enough with a few quick adjustments, but it may require a fair amount of time consuming tuning to have good power, driveability and efficiency.

As seen in this diagram, the rich side of stoichiometric ratio doesn’t have a big effect on the engine power, but it drastically increases fuel consumption.


Gasoline and air burn a chemically perfect ratio of 14.7 parts air to 1 part fuel, referred to as the stoichiometric ratio.  A carburetor is nothing more than a mechanical device that mixes fuel and air.  It is calibrated to do so at a particular ratio of air and fuel depending on the operating conditions.   That said, internal combustion engines are very tolerant to burning fuel ratios well outside the stoichiometric ratio, particularly on the rich side with little decrease in performance.   This is why one can bolt on an aftermarket carburetor with a generic calibration and the car will run ok, even if not near an optimal tune.

Here is an example of the many versions of the 4MV Quadrajet used by Chevrolet in 1967. Each of these have a unique calibration for each application.  Compare this to the aftermarket that makes one carburetor that covers many very different applications.


I overhauled my engine and performed some minor engine upgrades to my Ford Torino.  These modifications included increased compression and a more aggressive camshaft.  I also upgraded to an aftermarket vacuum secondary 4150 Holley.  It fired up and ran okay immediately upon install, but I knew to get my carburetor to run really well, I needed to do some serious tuning. My initial tune was done using traditional methods.  I set the carburetor idle settings with a vacuum gauge, adjusted the jetting by reading the spark plugs, and used trial and error to choose the correct vacuum secondary spring.   My carburetor was too lean out of the box, and my initial tune richened the carburetor up which significantly increased performance.

My Holley carb on my refreshed engine


The car was running well, but I knew I could do better.  I am far from a carburetor expert, but over the years I have developed pretty decent skills at tuning and rebuilding carburetors.  Nevertheless, my plug reading skills aren’t up to those of Smokey Yunick.  Thankfully today we can use modern technology to help tune a carburetor without Yunick’s skills.  An oxygen sensor connected to an Air Fuel Ratio (AFR) gauge will show precise measurement of the carburetors mixture at a particular moment in time. Adding a simple bung to the exhaust allows the use of this tool for a much more precise tune.

Holley carburetors are almost infinitely tunable, but how exactly do you use an AFR gauge to tune one?  To understand how to tune one, an explanation of the basics of the Holley’s carbs operation is in order.  The carburetor has several circuits, all of which need to be tuned to ensure proper fuel and air delivery.

First is the idle and transition circuit, often simply called the idle circuit.  This circuit controls the fuel delivery at idle, but also the transition period before the main circuit starts to pull fuel.  While many think that tuning the idle circuit only affects the idle, this is not true.  At very small throttle openings the engine is fed by both the idle jet and the transition slot.  This circuit is fined tuned with the idle mixture screws, screw in or out to lean or richen.  On some Holley’s the idle screws are only on the primary side, but others, have them on the secondary side as well for a total of four idle screws.  If the idle screws are unable to properly tune this circuit, this can be adjusted by changing the idle jet size or the idle air bleeds (more on air bleeds later).

The next circuit that comes into play is the main circuit.  This consists of the primary jets on the primary side of the carburetor.  Adjusting this circuit is simple, larger or smaller jets richen or lean this circuit.  An engine’s need for fuel will increase under heavy load situations, resulting in the primary circuit needing further fuel enrichment.  This is done by way of the power valve.

The power valve is held close by engine vacuum.  As the load increases on the engine and the vacuum drops, the power valve will open. This exposes a second set of jets, called the Power Valve Restriction Channel (PVRC).  Much like how a Quadrajet or a Carter AFB will pull a metering rod out of a jet to enrich the main circuit, the power valve opens up to expose the PVRC jets and enrich the main circuit.

The top picture shows that non-replaceable PVRC jets have to be drilled out. The lower picture shows screw-in replaceable jets.


PVRC jets are sometimes replaceable but on lower priced carburetors they must be drilled to increase in size. Like the idle circuit, the main jets also have air bleeds (sometimes replaceable), which can aid in tuning.

The top picture shows screw in replaceable air bleeds while the bottom picture shows non-replaceable pressed in air bleeds.


All carburetors have air bleeds or air correction jets which introduce air into the various circuits’ fuel mixture prior to discharge in the venturi. In the simplest terms, air bleeds suck air into a circuit to mix with the fuel.  The air bleeds become more effective as engine speed increases, meaning as more fuel is sucked into a circuit, more air in also sucked in.  This cancels out the tendency of the mixture to richen up with increased fuel demand.  On the main circuit, the air bleeds help tune the fuel curve.  If for example the fuel mixture is richening as the engine speed increases, using a larger air bleed will bring more air into the mixture as demand increases and which will lean out the mixture and flatten the fuel curve.  Not all Holley’s have replaceable air bleeds, but those that don’t can be tuned by drilling larger or using a small piece of fuse wire to partially restrict air flow.

Four barrel carburetors have a secondary side that only is used under high load and higher RPM situations.  The secondary side of a Holley carburetor looks very similar to the primary side, and like the primary side it uses two jets to feed the circuit.  The jets and air bleeds can be altered to deliver ideal fuel air mixtures under heavy throttle applications.  Unlike the primary side, the secondary side does not have any additional enrichment from a power valve.  Since this side of the carburetor is only used under high load situations and not under light cruise or light to moderate acceleration, there is no need to have smaller jetting with an enrichment circuit.  The only time this circuit is being used is when maximum performance is needed, so the jetting can be richer.  As a result, Holley typically calibrates the secondary jetting to be richer than the primary side because it does not have an enrichment circuit to add additional fuel.

This chart shows the different Holley Secondary springs that are available.


On a vacuum secondary carburetor, the secondary throttle blades for the carburetor are not opened by a mechanical linkage connected to the throttle cable.  A small spring loaded vacuum diaphragm opens the secondary throttle plates.  The vacuum diaphragm opens the secondary throttle blades based on the engine load, not the throttle input.  The vacuum signal from the primary venturi will increase with airflow.  After a certain point, it will allow vacuum into the diaphragm. This vacuum will eventually overcome the spring pressure and allow the secondaries to open.   Tuning of the rate of opening in crucial to optimal performance and is done so by way of changing the internal spring.  A too soft spring will allow the secondaries to open too quickly causing a bog.  A spring too stiff can cause them to open too late which compromises performance.

This graph shows the effect of different cams for the fuel delivery of the accelerator pump.


Finally the last area of tune is the accelerator pump circuit.  Sudden opening of the throttle on a carbureted engine, in particular those with high vacuum, results in the fuel falling out of atomization.  To overcome this, the accelerator pump shoots extra fuel into the carburetor to help compensate for this fuel loss.  This shot of fuel needs to be appropriate to ensure the engine doesn’t go too lean or too rich.  Holley has several areas that can be adjusted to get the pump tuned correctly.  The squirter size can be increase or decreased.  A smaller squirter will shoot a smaller fuel shot but do so over a longer period, while larger squirters will shoot more fuel over a shorter duration.  The pump is activated by a cam, and Holley offers numerous cams that alter the accelerator pump curve (see chart above).

This is the O2 sensor I installed which allowed me to use my AFR gauge. It can be removed and plugged when I am done tuning.


On my particular carburetor, I went through and re-tuned all these circuits with the AFR gauge. The AFR gauge clearly showed if a circuit is running too rich, too lean or at an ideal mixture.  Using my old school methods of tuning increased the engine performance and my fuel consumption. I discovered that I went too rich for most of my adjustments, which meant I could make significant efficiency improvements.  I used my AFR gauge with trial and error under various driving to help adjust each of the above circuits.  I was able to get my Holley to run at a much leaner, cleaner and more fuel-efficient cruise, and had no loss in performance. Even my wife commented that the exhaust smell had dramatically improved.

I temporarily mounted the AFR gauge in my ashtray so I could read it as I drove. I powered my gauge through the cigarette lighter. This allows for easy removal once the tuning is completed.


A carburetor will always be a compromise over a EFI system, but with and AFR gauge and some proper tuning you can have a pretty well-mannered machine that operates very well.  Finding a mechanic that can tune carbs today is about as rare as hen’s teeth.  Nevertheless, most mechanically inclined hobbyists can use this modern technology to drastically improve their classic car’s carburetor operation.