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According to EFI systems' marketing, carburetors are prone to cold start issues, poor fuel economy, and complicated upkeep. And while yes, they aren't as efficient as modern fuel management systems, they certainly aren't as bad as many make them out to be. A well-tuned carburetor can be every bit as streetable and enjoyable to drive as a car with a fuel injection system.

That isn't to say you won't need to learn your way around a carburetor, though. As the owner of a classic car, you will need to spend some time under the hood making adjustments to ensure the system is running right.

Don't worry. You don't need to be a rocket scientist to set a good baseline. Nor do you need to know the finer points of carburetor theory to keep your historic ride running. Plus, you're entering the ring with weighted gloves as The Drive’s crack team is ready to set you up with the knowledge you need to get that thing purring like a kitten.

Estimated Time Needed: 30 minutes to one hour

So, what exactly are we adjusting, and why are we adjusting it? To be frank, this article is all about making adjustments to your idle mixture and speed. The reason for this is because it's the one thing that most carburetors have in common. Even if they look different and handle wide-open throttle conditions in various manners, the majority rely on the same basic principles for adjusting idle running conditions—even the methods of making adjustments are very similar.

The primary goal of making these adjustments is to achieve the appropriate engine speed at idle while ensuring it runs as smoothly as possible. In doing so, you're establishing the proper fuel and air mixture at idle and setting an excellent baseline for further tuning.

Bear in mind that a carburetor supplies air and fuel to the engine differently at idle than it does at wide-open throttle. In short, separate systems and components come into play once you step on the gas pedal. They, too, will need dialing in. But before you get into that, you want to make sure that you have all of your idle adjustments in order as all systems come together to determine how the vehicle behaves in Park or Neutral and what it has to work with as you lean into it.

Your means of making these adjustments is with the external idle speed screw and idle mixture screws. Keep in mind the location of these screws will vary with each model of carburetor, so you’ll have to check your specific carb’s manual to find their exact location. For example, Holley Carburetors typically feature the idle mixture screws on the side of the metering block, while Rochester and Edelbrock's carburetors feature the idle mixture screws on the face of the carburetor.

The reason that you need to make adjustments is because the carburetor cannot. As temperature, altitude, and even the weather change, so does atmospheric pressure, meaning more or less air is available to the engine. Because the adjustments you make are analog, the amount of fuel going into the engine does not change with the atmosphere. So, you must adjust the carburetor to correct the engine's running condition at idle. Make sense?

Let’s get into it!

Unless you’re setting the baseline, the adjustments you’re making to the carb are performed while it’s running. While the pros never seem to mind, it’s still important to be mindful of the risks before getting started. Be sure to follow these safety tips throughout the procedure.

We can't say what you have on hand, but we can say that you don't need a lot to make adjustments to your carburetor. In fact, the only tools you really need are a screwdriver and your ears. But, if you're looking for an easier method that positively determines your engine's run condition as you tune, you might consider growing your collection.

Organizing your tools and gear so everything is easily reachable will save precious minutes waiting for your handy-dandy child or four-legged helper to bring you the sandpaper or blowtorch. (You won't need a blowtorch for this job. Please don’t have your kid hand you a blowtorch—Ed.)

You’ll also need a flat workspace, such as a garage floor, driveway, or street parking that’s also well-ventilated. Check your local laws to make sure you’re not violating any codes when using the street because we aren’t getting your ride out of the clink.

As much as The Drive loves to put the "you" in do-it-yourself, we know that not everyone has the proper tools, a safe workspace, the spare time, or the confidence to tackle major automotive repairs. Sometimes, you just need quality repair work performed by professionals you can trust like our partners, the certified mechanics at Goodyear Tire & Service.

Here are The Drive’s pro-tips on adjusting your carbs.

You’ve got questions, The Drive has answers!

A: One way to tell for sure is by "reading" the spark plugs. If the plug tip is white, the mixture is lean. If it's brown or black, it's rich. However, this method is usually linked to the carb's jetting and settings outside of idle, as these colorations come from part and wide-open throttle. At idle, a lean condition will bog down engine speed. If it’s too rich you can generally tell by a strong odor and/or smoke coming from the tailpipe. An AFR gauge is a great tool for getting the precise measurement of these conditions.

A: Simply put, poor running conditions are a dead giveaway. A rough idle, poor throttle response, and excessive fuel consumption are all signs to look for. If the carburetor was sitting for some time or hasn't been serviced in a while, you may need to clean it first.

A: Adjusting the mixture on a carburetor is done in several ways and the steps ultimately depend on the model of the carburetor you are working with. At idle, however, most carburetors simply require adjustments to be made with idle mixture screws.

A: Ensuring that the carburetor is adjusted correctly is a big step to take when reducing fuel consumption, but it's not the only thing you can do. Tuning the carb through the RPM range is also essential. Furthermore, because the system is mechanical, you need to practice driving habits that consume less fuel.

A: The best method to check air-fuel ratios is with an AFR gauge. Tuning by ear or with a vacuum port are effective ways to determine running condition, but don't actually read the air-fuel ratio. AFR gauges use an O2 sensor, just like modern cars, to provide an accurate and real-time reading. If they are within your budget, these tools are worth investing in for carb tuning.

WD-40 - 300134 Specialist Carb/Throttle Body & Parts Cleaner

AEM UEGO Air/Fuel Ratio Gauge

HFS Pressure & Vacuum Tester Gauge Test Kit

Craftsman Slotted Phillips Screwdriver Set

We’re here to be expert guides in everything How To related. Use us, compliment us, yell at us. Comment below, and let’s talk! You can also shout at us on Twitter or Instagram, here are our profiles. Got a question? Got a pro tip? Send us a note: guidesandgear@thedrive.com.

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