How to Adjust a Carburetor and When Is Tuning Needed?

Carburetors used to rule the streets before throttle bodies and multi-port fuel-injection systems were introduced. 

Emissions regulations during the 1980s forced auto manufacturers to phase out carburetors. However, there are still some vehicles that are equipped with them. 

Driving a vehicle with a carburetor will require you to tune several overlapping systems from time to time to ensure that everything will run smoothly. 

An Overview of Carburetor Operation

Carburetors are solely dependent on intake vacuum for fuel metering and delivery.

As the vacuum pulls down air through the carburetor’s throat, fuel is siphoned from the fuel bowl, which mixes with air to create a combustible mixture.

Carburetor engines have one or more idle ports just below the throttle plate where fuel passes through at idle speed. The idle mixture screw is used to fine tune fuel delivery at idle.

At higher engine speeds, metering jets pull the fuel that goes into the venturi. The air-fuel mixture then flows to the intake manifold and into the cylinders to produce power.

Carburetor vs. Fuel Injection

Unlike carburetor engines, fuel-injection systems don’t rely on intake vacuum.

Instead, fuel is directly transferred to the intake manifold, ports, or into the combustion chambers.

Electronic fuel injectors do a better job of atomizing fuel than carburetors, and gasoline direct injection (very high pressure) does an even better job of atomizing the fuel (smaller droplets for more complete combustion).

Mechanical carburetors can’t vary the air-fuel mixture automatically, requiring frequent adjustments to deliver the best performance under certain weather and road conditions.

Feedback carburetors began to appear around 1981 whereby an electronic control unit used O2 sensor input to regulate fuel delivery for the cleanest possible burn.

Meanwhile, electronic fuel-injection (EFI) systems rely on the powertrain control module (PCM) and several sensors to control the fuel injectors and other functions.

The Ideal Carburetor

A properly tuned carburetor should prevent the engine from stalling upon startup and when idling. It should also cause the throttle to deliver an instantaneous response as soon as the driver steps on the accelerator pedal.

Simply put, a good tune will help the engine achieve maximum horsepower and torque because the carburetor is able to deliver the best air-fuel mixture possible.

When you first tap the gas pedal, the choke should close. When the engine starts, a vacuum diaphragm called a “choke pull-off” or “vacuum break” will open the choke about one eighth of an inch. The choke is closed by a spiral bimetal spring that releases its spring pressure on the choke as it is heated, so that the choke is fully open within about 80 seconds or so.

Also, when you tap the accelerator pedal to close the choke on a cold engine, the accelerator pump also squirts fuel into the intake. Every time you accelerate, the same pump will deliver a squirt of fuel. If it doesn’t, the engine will stumble.

Fuel Injection engines deliver extra fuel during cold start by simultaneously firing all the injectors until after the engine starts, and then the injectors begin to operate sequentially with the firing order. Also, during warm operation, when the accelerator is pressed, the injectors provide extra fuel for the same reason the accelerator pump does on a carburetor.

Fuel Injection engines deliver extra fuel during cold start by simultaneously firing all the injectors until after the engine starts, and then the injectors begin to operate sequentially with the firing order.

– Richard McCuistian, ASE Certified Master Automobile Technician

Effects of a Mistuned Carburetor

The air-fuel mixture can either be too lean or too rich if the vehicle has a mistuned carburetor.

A mixture that’s too rich will cause the excess fuel to wash down the cylinder walls, dilute the oil, and damage the spark plugs.

Meanwhile, a mixture that’s too lean can affect the engine’s power and might result in detonation.

Symptoms of a Mistuned Carburetor

You might need to adjust your vehicle’s carburetor if you notice one or more of the following symptoms.

Hard Starting Engine

A hard starting engine is usually caused by a faulty choke mechanism and linkage.

A choke is a butterfly valve that reduces the carburetor’s air intake and draws fuel-rich charge into the cylinders during cold engine startup.

Chokes are sensitive components that require a simple adjustment or cleaning from time to time.

If the choke gets stuck in the “closed” position, it will suck in too much air and cause the engine to run rich.

Poor Fuel Economy

Poor fuel economy in a carburetor engine could be attributed to a misadjusted float inside the fuel bowl.

If the float is set too high, the fuel level could rise and cause the air-fuel mixture to get too rich. The same result can occur if the float gets saturated with oil.

Some carburetors with electronic feedback can also have the same problem if there are issues with the oxygen and coolant sensors.

Rough Idling

Air leaks between the carburetor and intake manifold can produce a rough idle condition and a lean misfire.

In some cases, this problem can be resolved by tightening the carburetor’s base bolts or replacing the gasket underneath the carburetor.

Rough idling can also be caused by a lean idle mixture adjustment or a dirty idle mixture circuit. The idle mixture can be adjusted using mixture screws. There is an idle stop screw that must be adjusted properly as well. If the idle stop screw is adjusted too high, EGR flow may happen at idle and cause a rough idle.

Hesitation or Stumble When Accelerating

A lean fuel mixture can cause the vehicle to hesitate or stumble when accelerating. Usually, this problem arises when the carburetor jets have been coated with fuel varnish deposits or the fuel bowl has been contaminated. A faulty accelerator pump can cause this, as well.

A carburetor cleaner can get the job done and restore the vehicle’s normal operation, but in some cases, rebuilding or replacing the carburetor might be necessary.

How to Adjust a Carburetor

Carburetor tuning can be overwhelming for newbie DIYers. This task requires the right equipment and automotive know-how and is usually done by professionals or drivers with enough experience in repairing and adjusting carburetors.

A person who understands carburetors can usually “wing it” by just tinkering with the adjustments. However, a person without understanding may end up doing more harm than good.

The job can range from tightening a few bolts to adjust the idle speed and idle mixture to resizing the main jets and tinkering with choke adjustments.

Adjusting a carburetor can also require major modifications, such as replacing venturis, boosters, emulsion tubes, metering blocks, discharge nozzles, and power valves, among others.

Carburetor Tuning Precautions

The steps on how to adjust the carburetor most likely begin with knowing its history. Some of the things you should expect before making the necessary adjustments include:

• Corroded plug circuits that restrict fuel flow
• Inoperative accelerator pump
• Leaking or heavy floats (causes flooding)
• Sticking or leaking fuel bowl inlet valves
• Clogged fuel filters
• Vacuum leaks
• Worn-out or damaged gaskets

Wrapping Up

Carburetors aren’t as advanced as electronic fuel-injection systems, which means you’ll need to put in a lot of work to make sure that the engine runs smoothly.

A carburetor that’s tuned properly can help your vehicle deliver the best performance possible, but a mistuned component can lead to several issues, including stalling, rough idling, and poor fuel economy, among others.

Lastly, adjusting a carburetor isn’t as simple as changing used engine oil or replacing a dirty air filter.

Carburetor tuning requires some extensive knowledge of automotive repair and shouldn’t be treated as a beginner-friendly task.

It’s always a good idea to seek help from a certified professional to ensure that all steps and protocols are followed correctly.

But if you think you can do the job on your own, you might want to consider conducting a detailed review of the carburetor’s history to know which parts need to get replaced or simply adjusted.

Where to Get Quality Replacement Carburetor Parts

Adjusting your carburetor might require a lot of different replacement parts such as boosters and emulsion tubes. If you leave these damaged parts as is, your carburetor likely won’t return to peak condition. Your vehicle will continue to show poor fuel economy, rough idling, and other symptoms of a mistuned carburetor. Thankfully, it’s easy to get your hands on replacement parts at CarParts.com.

At CarParts.com, we take pride in our wide selection of aftermarket brands and high- quality parts available at different price points for every budget. It’s our goal to help you order online with confidence, which is why we offer a lifetime replacement guarantee and implement a 60-day return policy. Our helpful and friendly customer service team is on standby around the clock to answer any questions or address any concerns you might have.

Whether you’re tuning your carburetor yourself or leaving it to  a trusted mechanic, don’t delay repairs longer than you need to. Order a new carburetor or other related parts at CarParts.com today.