How To Adjust Motorcycle Carburetor Air Fuel Mixture?


How To Adjust Motorcycle Carburetor Air Fuel Mixture

Knowing how to adjust the motorcycle carburetor air-fuel mixture is a crucial thing. The air-fuel combination of a motorcycle is also essential for the performance of your bike. A mixture that is too lean or rich will cause engine-related issues like poor acceleration, poor throttle response, and a lack of power. 

By seeing the complex engine terms, you may think of adjusting a motorcycle carburetor air-fuel mixture as a complicated process. But it is a relatively simple process if you follow a good guide and do the steps correctly.

The guide I am providing here is different. I have blended my experience and the copybook styles of adjusting the carburetor air-fuel mixture. That is why you will get an authentic feel while doing the steps in real life. Stay till the end of this guide, so don’t miss a bit of it.

Quick Guide To Adjust Motorcycle Carburetor Air Fuel Mixture: 6 Steps

In this guide, I will try to make the steps easy for you to adjust your motorcycle’s carburetor air-fuel mixture. You will just need a flathead screwdriver as equipment. You don’t need to be an A-grade student of the motorcycle engine to do the steps. But knowing some of the parts will be a plus.

Step 1: Warm Up The Engine

Don’t be surprised. Yeah. I recommend you do this thing while keeping your motorcycle alive. You can do two things to warm up your motorcycle engine. The first way is to drive your bike for a while, like 5 to 10 minutes. And the second way is to run the engine for five minutes.

After your motorcycle is warmed up, keep it on. The reason to keep the engine running is to assess the idling of your motorcycle engine while making adjustments. You will be able to evaluate the changes to the idling life, which is a plus of this method.

Step 2: Find Out The Air-fuel Mixture Screw

Now you have to find out the air-fuel mixture screw. There are two types of screws in a carburetor: the idling screw and the fuel screw. You have to work on the fuel screw to adjust the air-fuel mixture of the carburetor.

To find out the fuel screw, you have to find out the carburetor first. The part where the air filter is usually attached is the carburetor. Once you have found the carburetor, search for a brass screw on it. The screw is mostly slotted and flat-headed. After you have found the screw, move on to step 3.

Step 3: Rotate The Screw Clockwise

Here comes the use of the flat-head screwdriver. Take the screwdriver and start to rotate the screw clockwise. Or, you can simply call this process tightening the fuel screw. Tighten the screw till you hear the rough falling and rising sound than the usual idling sound.

Once you hear that unusual sound, stop doing your work with the screwdriver. Tightening the screw also means making the mixture lean. By doing so, the engine’s RPMs will be lower at idle.

Besides, you will need less fuel to run the engine. However, a too lean mixture is not suitable for your motorcycle engine and eventually causes damage to it.

Step 4: Rotate The Screw Anti-Clockwise

You have to go for the anticlockwise rotation if you are done with the clockwise rotation. In other words, now you are loosening the screw. You have to do one extra thing in this step. You have to count the rotation number from beginning to end.

Use the flathead screwdriver to do the anti-clockwise rotation of the screw. Stop until you hear the unusual sound, which should be like an aggressive revving sound. Count the number of spins and remember them or write them somewhere.

What loosening does to your engine is make the fuel mixture rich. If the fuel mixture is rich, the performance will be top-notch, but you will need more fuel to run the motorcycle smoothly. Too rich of a fuel mixture is not a good thing either.

Step 5: Place The Screw At A Specific Position

After doing steps 3 and 4, now you have the information regarding the position of the screw. Now you know what the peak tightening spot of the screw and the peak loosening spot of the screw are. What you have to do now is just to balance the position of the screw.

This is where counting the rotations at step 4 comes in handy. To balance the position of the screw, you have to rotate the screw clockwise again. Here, the number of rotations will be half the number of spins you counted in step 4. 

For example, if you went anti-clockwise by two complete rotations of the screw, rotate the screw fully for only one time. After doing this repositioning of the screw, you should be able to adjust the carburetor air-fuel mixture of your motorcycle.

Step 6: Do Some Finishing Tweaks

You have to constantly listen to the idling of your motorcycle while doing steps 3, 4, and 5. After completing step 5, you can do some minor tweaks if you are not happy with the idling sound.

For example, try rotating the screw both clockwise and anti-clockwise only ½ a turn. If you think rotation on a particular side of this measurement gives you the perfect feeling of the idling sound, you got things right. 

However, the experience you will get may not be the smoothest. If you are still unpleasant about the incident, try calling an expert.

5 Things To Keep In Mind While Adjusting Carburetor Air Fuel Mixture

Following a helpful guide can make you save the billings of a professional. For this very reason, you have to act like a professional also. The following are the four things to keep while adjusting the carburetor air-fuel mixture.

1. Don’t Touch Anything Bare-Hand:

The first thing you need to keep in mind is that you are adjusting the carburetor air-fuel mixture when the motorcycle is alive. If the bike is active, some engine parts will be scorching. 

You have to be extremely careful to not accidentally touch anything on your motorcycle’s engine barehand. This can cause severe burn damage to your hand tissue. I think you don’t want anything like this to happen to you. So, don’t touch anything without protection.

2. Find Out The Right Screw Before Starting The Adjustment:

There are two screws on a carburetor. One is the idle screw, and another one is the fuel screw. You should know which one you have to find by seeing the names. It should be pretty easy to find the fuel screw. The fuel screw is usually the golden brass screw on a carburetor. 

3. Don’t Over-tighten Or Over-loose The Air-fuel Mixture Screw:

You must rotate the fuel mixture screw while adjusting the carburetor air-fuel mixture. Besides, you also have to carefully hear the idling sound of the motorcycle while performing those rotations with the screwdriver. 

The moment you hear any unusual idling sound, you should stop. If you do not control and proceed further, you might damage your motorcycle. 

Because in that case, the adjustment won’t be correct, and the mixture will be either on the rich side or the lean side. Neither of them is suitable for your motorcycle. That is why you should not over-tighten or over-loose the screw.

4. Ask For Help When Necessary:

I am repeatedly saying this. This method is challenging to accomplish if you are a total beginner. You may fall into awkward situations or situations that are not covered in this article. It can happen. 

I am trying to cover most motorcycle models by showing the standard method. There can always be exceptions, right? If you happen to fall in one of the mentioned circumstances, please call a professional. 

Yeah, it might cost some money from your pocket. But if you don’t call, you may even need to spend a fortune to make things right!

5. Don’t Experiment On Anything That You Don’t Know Properly:

Some people love to try things or do improvisations. Hey, if you are one of them, it is a disclaimer. Never try something yourself if you do not have accurate information regarding the consequences. 

Experiments are only for experts, but the investigation is only after they are happy with their paper-based research works. Instead of saving some bucks or doing something exciting, you may experience something horrific. So, say no to experiments.

7 Tips And Tricks To Tune The Carburetor For Better Performance

The carburetor is responsible for getting fuel into the engine and distributing it evenly throughout the intake manifold. If you’re looking for ways to improve performance in your car, you can tune up your carburetor. Here are my seven tips on how to do so:

Tip 1: Clean The Carburetor

Before turning the carburetor, you should clean it thoroughly using a suitable cleaning agent such as Seafoam or Regane® Complete Fuel System Cleaner.

This is to remove any deposits on the inside of the carburetor that could be causing fuel flow and performance issues. It also ensures that you have a clean surface to work with. Things will be smooth while adjusting the various settings on your carburetor like air-fuel mixture, idle speed, etc.

Tip 2: Adjust The Air-fuel Mixture 

The air-fuel mixture determines how much fuel gets injected into the engine during each combustion cycle. It also determines how much power the motor will accumulate due to those fuel injections. 

Neither too rich nor too lean a mixture is suitable for your motorcycle. Adjust the mixture to your need to enjoy it to the fullest.

Tip 3: Adjust The Float

The float valve in a carburetor regulates fuel flow into the carburetor and throttle body. You can adjust it by turning a screw-on top of the float valve until it’s positioned correctly. 

You should also check if there are any cracks or leaks in your float valves. Replace them with new ones if you think the old ones are not functioning.

Tip 4: Adjust The Main Jet

If you have a large main jet installed in your carburetor, it’s time to replace it with a smaller one. The main jet controls how much fuel enters each cylinder. 

So, if there’s too much fuel entering into each cylinder, you’ll have plenty of power at high RPMs. But there won’t be enough torque at lower RPMs. A smaller jet easily solves this problem.

Tip 5: Clean The Air Filter

The primary purpose of an air filter is to filter out dust and dirt particles from entering the engine system. Cleaning the filter ensures that there is no extra dust or dirt that can affect your engine’s performance. It also helps prevent the clogging of filters, leading to reduced fuel efficiency and poor performance.

Tip 6: Replace Old Spark Plugs

Spark plugs help ignite fuel to burn entirely inside an engine cylinder. Old spark plugs can cause misfires which lead to incomplete combustion. It causes poor performance and high emissions from exhaust pipes. Replace the old spark plugs with the new ones to solve the issue.

Tip 7: Adjust Vacuum Lines

Vacuum lines provide vacuum pressure for better control over various components such as fuel pumps, throttle valves, and idle speed control valves. If there are leaks on these lines, they may not function properly. 

Thus, it will affect the overall performance of your motorcycle. So, get the vacuum lines adjusted as soon as possible.

Related Questions:

How Do You Adjust The Air Fuel Mixture Screw On A Motorcycle?

To adjust the air-fuel mixture, screw on your bike, turn it clockwise to lean your fuel mixture, or counter-clockwise to richen it up. The exact amount of change depends on how far you turn it and what type of bike you have.

The air-fuel mixture screw is the brass screw at the bottom of your carburetor that you can turn with a flathead screwdriver. It’s usually located on the side or back of your carburetor, but sometimes it’s in front.

How Do You Fix A Rich Air-fuel Mixture On A Motorcycle Carburetor?

If your motorcycle is idling rough without valid reasons, it may be because of the rich mixture. The rich air-fuel combination means the fuel screw is looser than it was supposed to be. 

To fix a rich air-fuel mixture on a motorcycle, tighten the fuel mixture screw. You have to tighten the screw up to a level where you don’t hear the idling sound rough. I recommend you adjust the screw properly by following a good guide.

How Do You Know If Your Carburetor Is Rich Or Lean?

If your motorcycle is running rich, it will be hard to start, have poor acceleration, and have high exhaust emissions. It may also stall at low rpm or idle poorly with a lot of smoke coming out of the exhaust pipe when idling. A lean carburetor will cause your bike to run poorly under load, especially at high rpm. 

This issue is that not enough oxygen gets into the combustion chamber for complete combustion to occur. This causes poor performance, reduced fuel economy, and higher emissions from incomplete combustion in your engine’s cylinders.

Which Way Do Idle Motorcycle Carburetor Air Mixture Screws Turn?

Idle air screws usually turn clockwise on most motorcycles to increase airflow through the carburetor and lean out the mixture. They rotate counterclockwise to increase airflow into the engine and make it run richer (more fuel).

Some bikes have different configurations. So, check your owner’s manual for specific instructions for your make and model of motorcycle. Inspect before making any changes to these settings.

Final Verdict:

Adjusting the motorcycle carburetor air-fuel mixture becomes necessary when you feel something is not right with your motorcycle’s idling and overall performance. I have told you guys how to adjust the motorcycle carburetor air-fuel mixture to eliminate the issue. 

Although it is a relatively straightforward process with the right guide, always be careful regarding the steps. I hope you can adjust the air-fuel mixture on your own by following my guide. Have a good day. 

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Ahsan Ullah

Hi guys, This is Ahsan Ullah. I have been riding motorcycle for more than 15 years. In this long period, I have experienced a lot of things about bike riding. Now I am trying to share my experience with you on this website.

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