How to Adjust Air Fuel Mixture Screw

How to Adjust Air Fuel Mixture Screw

The air-to-fuel mixture of any motorcycle engine will dramatically affect the bike. In some cases, you may want a richer mixture, while at other times, it may be a better idea for the engine to run a little more lean. Whether for performance, fuel efficiency, or a mix of both, there are plenty of reasons to adjust the amount of fuel being delivered by the fuel system. 

Fortunately, for carbureted bikes, this process is made easy by design, so adjusting the carburetor just takes a little know-how and a screwdriver.

We’ll take a look at how to adjust the air-to-fuel mixture on your bike, and go over a few things that every rider should know about air and fuel mixtures so that you will be able to make the most of your own adjustments.  

How to Adjust the Air-Fuel Mixture Screw

Carburetors are a purely mechanical method of fuel delivery and, as a result, are generally easy to adjust. Because the carburetor isn’t controlled electronically, the amount of fuel that is delivered to the engine is manually adjusted by adjusting certain components. The one we will be dealing with specifically is called the “fuel-screw” or the “air-screw” if you’re working with a 2-stroke engine.

This is a brass, slotted screw usually located on the side of the carburetor that you will tighten or loosen with a flathead screwdriver. 

How to Adjust the Air-Fuel Mixture Screw

The function of this screw is to control the air-to-fuel ratio of the engine. This is an important step, as air and fuel make up half of what any engine needs in order actually to produce power. When combined with compression and spark, air and fuel produce the explosions that ultimately result in your bike moving forward.

By adjusting the mixture of air and fuel, we can influence the amount of power that the engine produces, potentially resulting in a quicker bike without changing parts. 

Making this adjustment comes down to turning the screw on the carburetor to allow more or less fuel through the system, either enriching the mix or leaning it out. Aside from power, the engine’s smoothness, or even its ability to function at all, will be affected based on how rich or lean you choose to make the mixture.

On a “fuel screw,” turning the screw clockwise reduces the amount of fuel in the system, and turning it counterclockwise allows more fuel through, which leans or richens the mixture, respectively. 

On a two-stroke engine, the screw will affect the amount of air instead. A clockwise turn limits the air and richens the mixture, while turning the screw counterclockwise increases the air in the mixture, making it leaner. 

While actually making the adjustment is very simple, there is more to the changes you make than just turning the screw.  

Things to Keep in Mind When Adjusting the Air-Fuel Mixture Screw on Your Motorcycle

We’ve already gone over a little of what making this adjustment will do for your bike and why you may want to make the adjustment in the first place. Here we will go over some of the things to keep in mind and some things that should always be standard practice beyond simply turning the screw. 

Always warm up the engine.

When adjusting the air-fuel mixture, you should always ensure that the engine is running at its typical operating temperature, as adjusting a “cold” engine will produce less consistent results. Let the bike idle for around five minutes before making your adjustments. Letting the bike idle will have it reach normal operating temperatures, and your adjustments will be much more consistent and predictable.  

Warming the engine

It’s not just about power.

There are numerous reasons to adjust the air-fuel mixture on your bike beyond just making more power. These can include:

  • Fuel Efficiency
  • Elevation
  • Smoothness
  • Reliability

The chemical reaction that happens when an engine goes through its operating cycle is one that can be affected by everything from the quality and condition of the engine components to where you are riding. A bike that is running at a higher elevation, for example, will be working with thinner air and therefore require more fuel to operate effectively than it would at sea level. 

The right fuel-to-air mixture can also help the bike go longer between certain maintenance procedures, like changing the spark plugs and keeping the engine running more smoothly. 

If you’re concerned with your fuel economy, a leaner mixture will use less fuel, and if carefully adjusted will result in a smaller impact on the performance of the engine as well. 

How to Tell if You’ve Gone Too Far

Running an engine too rich or too lean will have a negative impact on the way the engine operates. A mixture that is too rich can foul up the spark plugs or even flood the engine so that it doesn’t start at all. If the mixture is too lean the engine may suffer severe power loss, or not be getting enough fuel to run at all. When adjusting your air-to-fuel mixture, you will be looking for a comfortable medium. 

Lifted front wheel of the dirt bike

When you increase the fuel going to the engine, listen for when the idle begins to sound irregular. This will typically be an inconsistent and rushed tone that is distinct from what is expected from an idling engine.

A mixture that is too rich will also be accompanied by the scent of unburnt fuel in the exhaust. If you encounter either of these signs, note the position of the screw and dial it back until the engine is running smoothly once more.

When reducing the fuel to the engine, you will be listening for a rough exhaust note in the engine. This will usually sound almost like the bike is “choking” and is a burbling up and down sound as the fuel struggles to ignite.

When it is running this lean, the engine will be hesitant under power or even cut out when the accelerator is applied if you reach this level, note the position of the adjuster screw and increase the fuel delivery (or decrease the airflow with a 2-stroke engine). 

Additional tools

In addition to making the adjustments on what you hear and sometimes smell for yourself, there are additional tools that can help make the process a little easier, or a more technically minded rider may have a specific ratio that they want, and in that case, they will need to know more precise information. 

Garage tools

An emissions testing machine is a device that can provide more exact readings on the air-fuel ratio by reading the exhaust’s CO levels. For instance, 0.38 percent CO translates to 13:1 air/fuel. By using an emissions testing machine to read the CO level of the exhaust, you will be able to make precision adjustments, which is useful if you are attempting to match the factory recommendations for tuning the air-to-fuel ratio. 

Frequently Asked Questions

How do you tell if a carburetor is rich or lean?

If the engine is running rich, this means that it is getting too much fuel. Listen for the engine rushing, like it is revving up, during idle. You will also smell unburnt gas in the bike’s exhaust much more strongly than if the engine is running normally. 

A lean engine is not getting enough fuel and will run rough and irregularly. Listen for a rising and falling burble that may even sound like the bike is “choking.” Applying the accelerator will have a delayed response or may even cause the bike to shut off as well when the engine is running too lean. 

How do you balance the air-fuel ratio?

Balancing the air-fuel ratio is about finding the spot where the engine runs most effectively. Taking note of where the bike begins running too rich and too lean will give you a range of where a balanced mixture will be when adjusting.

Making basic adjustments will get you to a good approximate balance, but if you are aiming for a specific air-fuel ratio, you will need tools like an emissions machine to provide more precise information about your mixture. 

What is the best air-fuel ratio for a carburetor?

The “best” mixture for a carburetor can be something different depending on what you are attempting to achieve with your adjustments. An air-fuel ratio optimized for performance will be dramatically different from one that is meant to improve fuel economy. Most motorcycles will have a factory-recommended air-fuel mix, which is a good default to aim for when making adjustments with no special goal in mind. 


In this article, we’ve taken a look into adjusting your carburetor and learned that this one-step process is a bit more involved than just turning a screw. Knowing how to tell when the engine is running rich or lean and what these two things mean is a valuable bit of information that you will find helpful when tailoring your own bike to better suit the way you ride. 

Regardless of whether you’re aiming for a high-performance tune for the track or an efficient, road-friendly ratio that maximizes your fuel economy, always keep this information in mind, and you’ll have a much better experience when dialing in your preferences.

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Ryan Christian

My lifelong love of everything on wheels began with a dusty old scooter and a set of second hand wrenches. Since then I’ve spent every moment I can spare finding new dirt paths, winding country roads, and long open highways. I write to share my passion with other enthusiasts, and maybe inspire one or two new ones along the way!