How to Adjust Air Fuel Mixture Screw in 5 Easy Steps? 

An air-fuel mixture screw of a carburetor regulates the amount of air that mixes with this fuel. Once adjusted, the screw can change the engine’s idle speed and running smoothness. We will show you how to adjust the air-fuel mixture screw. This six-step guide is easy to follow and straightforward to get your job done. 

Let’s read on to discover! 

How To Adjust Air Fuel Mixture Screw?

How to Adjust Air Fuel Mixture Screw

You don’t have to be a professional mechanic to make adjustments to the air-fuel mixture screw. The six steps below will help you. 

Step 1: Warm up the engine

You can start your vehicle by turning the key or pushing an ignition button. Give it about 5 minutes to warm up while it is idling.

Please note that the engine should be working when you try to adjust the air-fuel mixture screw because the running engine allows you to hear how your adjustment affects its speed. 

Step 2: Locate the air-fuel mixture screw  

Your air filter, which is often circular as well as cone-shaped, is next to the carburetor. Generally, it rests atop the carburetor. 

Then, you need to find the fuel screw. If you see a slotted, flat-headed, gold screw on your carburetor, it must be what you are looking for. 

Step 3: Tighten the screw

When the engine starts to sound harsh, turn your air filter screw clockwise using a flathead screwdriver.

This step will give you the lean air-fuel mix and less gasoline reaching the engine. As a result, an engine idles at a lower RPM.

Your vehicle will use less fuel to operate efficiently if left alone. The increased friction from moving components and the higher operating temperature will harm your vehicle.

Step 4: Loosen the screw

Turn the screw counterclockwise to loosen it, counting the spins you make until this engine produces weird sounds.

A richer air-fuel mixture is achievable by releasing the screw as fuel enters the engine. These engine idles at a higher RPM as a result. Your car will use more fuel than it should, raising your fuel costs.

Step 5: Set the proper position for the screw

The two steps are for testing how loose or tight the screw can be. Both are harmful to the engine, so you need this final step to make a perfect adjustment. 

Turn the mixture screw with a flathead screwdriver about halfway between the place the idle sounds rough and irregular.

Until it sounds most even and clean, turn the screw a half turn either anticlockwise or clockwise if you aren’t pleased with this sound. This process is to balance the idle mixture. 


1. Does the air-fuel mixture screw only affect the idle?

The air screw will mainly affect the mixture at idle. Besides, the speed and smoothness of an engine’s idle can also change by turning this screw. 

2. What is the purpose of adjusting your fuel-air mixture?

The primary purpose of adjusting the fuel mixture is to reduce this fuel flow to compensate for lower air density.  

3. What causes a high fuel-air ratio?

This problem often comes from a faulty signal of an engine getting colder than it actually is. The malfunctioning oxygen sensor sends the wrong signal to the engine monitor, resulting in a high fuel condition.   

4. What are the symptoms of a rich fuel mixture?

Signs of a rich fuel mixture include clogged spark plugs, terrible engine performance, and black exhaust.  


You can easily adjust the air-fuel mixture screw by locating it in your engine and turning it. The key is to listen to the engine’s sound. It should be smooth and even for the best performance. 

Hopefully, you will find this guide helpful. Please let us know if you have any problems when trying those steps. We are always ready to help. 

Thank you for reading! 

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