How to Adjust Air Fuel Mixture on a Motorcycle Carburetor

How to Adjust Air Fuel Mixture on a Motorcycle Carburetor? Easy Guide in 2024

Carburetors have an air-fuel mixture screw that regulates the amount of air injected into the mixture before it enters the combustion chamber. It is possible to alter the idle speed and smoothness of an engine by adjusting this screw.

It doesn’t matter whether you’re changing an air-fuel mixture screw on a vehicle, motorbike, or any other form of a tiny engine; to get the unwavering performance, you need to know how to adjust air fuel mixture on a motorcycle carburetor

While the engine was running and warmed up, make any necessary changes. A well-balanced fuel-air mixture is achieved by turning the screw until the engine idles smoothly with no rough or irregular noises.

Let’s Get into the Adjustment Process

Let's Get into the Adjustment Process

Step 01 – Warm-up

To get the engine warmed up, turn it on and allow it to run for five minutes at a time. Start the engine by turning the ignition switch. Let the engine idle for approximately 5 minutes to get it up to operating temperature. After the engine has warmed up, keep it running.

If you’re going to fiddle with the air-fuel mixture screw, do it when the engine is warm and running so you can hear how it affects the idle speed. Any engine having an air-fuel mixture screw will follow the same procedure. Anything having a carburetor is eligible for consideration.

Step 2 – Locate the Carburetor

To locate the carburetor, look for the air filter on the engine. A circular or cylinder air filter may be seen on the engine, so keep an eye out for it! It is to the carburetor that the air purifier is connected to the engine.

If you have an automobile, the air filter may be huge and circular. The carburetor is normally where it rests. It is common for the air filter on a motorbike to be located on either side of the motor and facing backward.

Step 3 – Locate Gold Screw

You’ll need to locate the carburetor’s gold-colored brass screw with a flat head. Find the gold screw with a flat slotted head on the carburetor by looking at all the other screws on the carburetor. This is the air-fuel ratio adjustment screw.

In general, air-fuel mixture adjustments are found on the carburetor’s side; however, this varies by engine type.

Step 4 – Screwing

When the engine starts to whine, slowly reverse the direction of the screw. This can be easily accomplished with a flathead screwdriver. When the engine’s regular idling sound changes to a harsh rising and falling noise, it’s time to switch off the engine.

As a result, a tighter screw reduces the quantity of gasoline going into the engine, resulting in a less efficient combustion process. Engine idle speed is lowered when the screw is tightened, which is also referred to as rendering the fuel mix leaner.

It is possible to use a low fuel mixture and yet have the engine perform at its best. Motors are susceptible to damage as a result of increased heat and friction between the various moving components. Don’t forget to check the How to adjust a carburetor that is running rich?

Step 5 – Loosen the Screw

Turn the screw loose till the engine sounds erratic. Turn the bolt counterclockwise with your flathead screwdriver, keeping track of the number of revolutions you make. Stop moving the crank if it sounds like the engine is revving too quickly.

Easing the bolt enhances the fuel and air mixture combination and surges the quantity of gasoline smooth into the engine. Keeping the fuel system richer, or loosening the screw, raises the idle RPMs, which is what is meant by “loosening the screw.”

When using a high-octane fuel combination, the engines need a lot more gasoline than they should in order to work at their maximum efficiency. However, despite this, the engine will operate at a lower temperature and with greater power. You also check our guide for How to Remove Throttle Cable from Carburetor?

Step 6 – Set the Screw

Make sure the screw is centered in the area that sounds harsh and erratic. Using a screwdriver, turn the screw counterclockwise until the engine’s idle sounds somewhere between irregular and harsh. If you do this, the engine will be set to a constant idle speed.

Final Step

Adjustments should be made half a turn in each direction is all that is needed to get the best idling speed. Observe how the idle sounds by turning the screw around and clockwise half a round from the center position. Make sure that the engine’s idle is as uniform and smooth as possible by adjusting the screw to the desired setting.

If the engine’s sound becomes louder or more uneven after a half-turn in either direction, you may simply restore the bolt to its initial position. This process of adjustment is also known as “idle mixture balancing”.

Most engines operate best at an air-to-fuel ratio of 14.7:1. If you’re tuning a high-performance racing vehicle or motorbike, you’ll want to be as accurate as possible when determining your engine’s AFR, but this isn’t always essential.


1. Two-stroke engine fires once, then won’t fire. How many turns out should the mixer screw be out?

Common practice dictates that the adjustment screw be turned in two complete turns and then back out on most normally aspirated engines. Most of the adjustability range may be reached this way. In order to make precise adjustments, the engine has to be running and warmed up first.

The sort of carburetor you’re using makes a difference. It makes a tremendous difference whether it has only one neck or many as four. One adjustment screw is found on single barrels; however on multi-barreled devices, there is normally one for each pair of two adjusters. Find out by screwing the adjuster into its fullest position. Then, do a full turn backward. In most cases, this places the slider in the middle of its range of motion. Getting started is straightforward once you’ve reached that point.

2. If the mixture screw is turned clockwise, is the fuel-lean or rich?

The fuel flow is reduced, and the engine runs lean when the adjuster is cranked clockwise. For those who recall, when the fuel flow is restricted, it is increased when the screw is loosened, as is the case in this example.

You will notice too much fuel all the way in. When the engine is running, it will split too much gas underneath the air cleaner cover.

When this happens, it means the accelerator pump has failed. It’s only possible to feed the engine if you don’t have any flow in the mixture. Feed jets and butterfly valves in the carburetor boost fuel flow as the throttle is advanced. The vacuum formed by the combustion is drawing gasoline via the jets if the throttle pump is open.

3. What’s the problem when I have to pull the outboard so many times before it starts?

In many cases, this is an indicator that there is a lack of fuel. If the engine has been stored for a significant amount of time, it will need to be primed before use. When the compression level is achieved, gasoline evaporates, which makes it impossible to start the engine.

4. How do I check the air-fuel mixture if my vehicle doesn’t have a carburetor?

The computer of a modern-day car controls the air-fuel mixture adjustment. Emission testing requires a five-gas analyzer, although you can easily acquire an air/fuel mixture measurement using a scanner.


The motor that powers your motorcycle is a sizeable component of the machine. If it isn’t performing properly, it might cause a whole host of problems. Maintaining your motorcycle in excellent condition is essential if you want to get the most use out of it. If you are having any problems with your bicycle, you should contact a technician as soon as possible. You will not be required to invest as much as you would if you attempted to remedy the problem on your own.