How to Adjust a Carburetor

adjusting a carburetor while reading vacuum gauge
Adjusting a carburetor can be achieved with even the simplest tools. Tim Stocker Photography / Getty Images

A carburetor, or “carb,” is an extremely reliable and simple machine, and to this day it functions just as it did when originally built in 1888 by Karl Benz, inventor of the first automobile. All intake air passes through the barrels of one or more carburetors. As the air passes through the venturi, it speeds up, creating a low-pressure zone. Fuel is pulled into this low-pressure zone and distributed to the cylinders for combustion.

While modern cars use electronic fuel injection, muscle cars, race cars, motorcycles, and most small power equipment and power toys, use carburetors to blend air and fuel as they enter the intake.

carburetor cutaway and flow
Here, we can see various parts of a simplified carburetor. Intake air (blue) flows top to bottom through a venturi, where low pressure pulls fuel (green) into the air-stream and past the throttle plate (brown). Dorling Kindersley / Getty Images

Exactly how much fuel is pulled through the carburetor depends on several factors, such as air temperature, barometric pressure, choke opening, throttle opening, idle air bypass opening, idle jet adjustment, and fuel jet size. Two- and four-barrel carburetors feature individual adjustments for fuel mixture. When it comes to tuning or adjusting a carburetor, the aim is to balance the air-fuel ratio, the mass of fuel delivered per mass of air, balancing power output, fuel economy, emissions, and engine longevity.

Air-Fuel Ratio

The ideal chemical air-fuel ratio, in which all the fuel is oxidized, is 14.7:1, that is, 14.7 parts air to 1 part fuel. A “rich” condition means more fuel is being used, an air-fuel ratio less than 14.7, while a “lean” condition means less fuel is being used, an air-fuel ratio higher than 14.7. For best power, most engines run rich, 12.5 to 13.5 at full-throttle. For best part-throttle low-load cruising fuel economy, engines typically run higher than 15. Proper balance is critical, however, as running higher than 14.7 in full-throttle high-load conditions could lead to engine damage.

How to Adjust a Carburetor

a burnt piston
This type of destruction could be caused by a poorly-adjusted carburetor, running too lean in high load driving. Bukk / Wikimedia Commons

There are a couple of ways to adjust a carburetor. There’s the time-tested trial-and-error method, which is subjective and based on how the vehicle “feels” to drive, dedicated track time and a consistent driving pattern, as well as reading intake vacuum, spark plug condition, exhaust smell, and engine operation. The scientific method is a more accurate procedure, and includes the use of air-fuel ratio sensor feedback, exhaust gas analysis, and a dynamometer.

parts of a typical single-barrel carburetor
Know your way around the carburetor. Scheinwerfermann / Wikimedia Commons

When adjusting a carburetor, it’s important to start at a known baseline, perhaps the stock setup. You may need to make some initial rough adjustments to get the engine to run at all, then move on to fine-tuning once the engine is at operating temperature.

Warm up the engine to operating temperature and check the float level. If the fuel in the bowl is higher or lower, air-fuel ratio will be affected. High fuel level will lead to a rich condition, while low fuel level will lead to a lean condition, both of which are undesirable. Adjust or repair the carburetor and fuel delivery system to ensure consistent fuel level.

Adjust the idle-mixture screw to maximize idle speed, then adjust the idle-air bypass or idle speed back down to a smooth 600-800 RPM. You’ll have to make multiple adjustments back and forth to get the right idle-mix / idle-speed combination. On multi-barrel carburetors or multiple carburetors, be sure to adjust all idle-mix screws and idle-air bypasses the same amount, usually a quarter-turn per adjustment. Wait a minute or two after each adjustment to stabilize engine operation. Richen the idle-mix if you experience off-idle stumbling.

Adjusting jets and needles is where you will really get intimate with your carburetor, because this requires disassembly to get to the heart of the carb. Be sure to work in a clean area to prevent contamination and lost parts. “Dial-a-Jet”-type carburetors allow external adjustments to primary and secondary fuel mix adjustments, but most carburetors require disassembly.

dyno testing
Dynamometer testing can tell you how your carburetor adjustments affect performance, fuel economy, and emissions. shaunl / Getty Images

Again, the key to adjusting a carburetor is small steps, followed by repeatable feedback, whether it’s an air-fuel ratio sensor or the track timer. We suggest the scientific method and adjusting for feel but work with what you have. When adjusting jet sizes, bigger jets richen the mixture, smaller jets vice-versa. Most experts suggest jumping two jet sizes to rough-in on the right air-fuel ratio, then single jet sizes to fine-tune.

For carburetors with primary and secondary circuits, be sure to adjust and test individually, making the same changes and going through the same test procedures after each adjustment. Note, if you get more than six jet sizes away from the stock setup and you still can’t achieve the correct air-fuel ratio, you might have another problem with the carburetor, fuel pump, intake, cylinders, or ignition.

Tune That Carburetor Again

American muscle car engine with multiple carburetors
With multiple carburetors, be sure to tune each carburetor with the others for best cylinder balancing. Brian Stablyk / Getty Images

Finally, because the carburetor is a fixed fuel metering device, it cannot adjust for different driving conditions, weather changes, fuel deposits, or engine wear. If any of these things change, you’ll need to adjust the carburetor to match the new conditions. Failing to adjust the carburetor could lead to reduced power output, poor fuel economy, higher emissions, or even engine damage. That’s why modern vehicles use electronic fuel injection to achieve unprecedented power, fuel economy, and emissions numbers.