How To Adjust Carburetor Air Fuel Mixture On Motorcycle

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Believe it or not, you can adjust how much fuel your bike uses and how powerful it is. This is done by adjusting the air-fuel mixture on the motorcycle’s carburetor. You can’t change your engine but you can change how it runs. If you want to take full control of how well the engine performs, you need to adjust the fuel mixture screws. Here’s how to do that.

  1. Warm-up engine for 5 minutes
  2. Locate carburetor mixture screw
  3. Tighten the screw
  4. Unscrew and count the turns
  5. Find the spot between rough and irregular
  6. Make adjustments

It’s easier said than done. Many beginners don’t even think about making a fuel mixture adjustment because of the fear of messing things up. Carbs are important for starting bikes and maintaining the air-fuel ratio. If you’re going to be doing this, read the step-by-step guide thoroughly. How to adjust the air-fuel mixture on motorcycles.

Prepare Your Engine

Adjusting the carb mixture should be done on a hot engine. Cold engines have a different air-fuel ratio than hot engines. For you, it’s important to adjust the mixture on a running, warmed-up engine.

Just start your bike and let it idle for at least 5 minutes. Do not turn off your bike. You need to listen to how your bike sounds as you are adjusting the screw. You will want to hear how it sounds with the throttle pushed too.

Also, you may want to do this in a secluded area. Somewhere where you won’t disturb any neighbors. The thing is, you will be revving your engine quite a lot, so you will make a racket. If you have problematic neighbors, believe me, you are better off doing this somewhere else.

Locate The Mixture Screw

Locate The Mixture Screw

First of all, you need to locate the carburetor. I think most of you already know where it is, but I have to assume that there are beginners here. The easiest way to locate the carb is by finding out where the air filter is. 

The air filter should be quite visible. It’s attached to the carb, that’s how you know where the carb is. This is the same for all motor vehicles like cars, bikes, etc. The carb holds the air filter for the engine. On bikes, you can find the carb next to the engine, it’s facing towards the back wheel.

Now you need to find the screws. Normally, there are three screws on carbs, the air screw, the fuel screw, and the idle speed screw. The names say it all. The air screw controls the air mixture. the fuel screw the fuel mixture, and the idle screw controls the idle mixture.

The location of the screws depends on the type of engine you have. The air screw and the stock fuel screw are usually located side by side. The idle screw is on the top of the carb, away from the engine.

Adjust To A Lean Mixture

Once you’re done locating the screws, get a screwdriver and you can start adjusting. This is a complicated process, so remember to be careful.

Tighten the fuel mixture screw by turning it clockwise. This will decrease the amount of fuel getting in, i.e, making it leaner. Turn the screw clockwise, stop when the engine starts sounding rough.

Running on a lean mixture makes your bike idle at lower RPM. With the default settings, your bike should be smooth while idling. But when you lower the RPM at which it idles, it will start to sound rough. As you may have noticed with your bike.

Adjust To A Richer Mixture

You don’t want your bike to be running on a lean mixture for a long time. A rough-sounding engine means that there is not enough fuel. This also means that there is a lot of friction between parts which leads to damage.

Now you are going to be loosening the mixture. Again, with the screwdriver turn counter-clockwise. You want to stop somewhere before the engine starts sounding really off like it’s revving. But as you’re doing that, you need to count the turns you made. This is crucial.

The ideal mixture you are looking for is somewhere between these two extremes. Keeping count just makes the process easier. You can do this by ear but counting is easier.

Find The Middle

Find The Middle

Did you keep count? How many counter-clockwise turns did it get? You are looking for that normal idling that your car used to have. Suddenly, you are getting cold starts. Or, maybe you have noticed that something is off with the fuel usage.

Well, finding the middle between a rough and a revving engine should fix your problems. While the issues with your engine may not be related to the carb, tuning the air-fuel mixture is a way to solve them.

To find the middle, turn the screw clockwise again. If you took full three clockwise turns, now you need to the one whole clockwise turn followed by a half turn. Get it? Finding the middle. If you did this right, your engine idling should sound normal.

Final Adjustments

You may have hit the spot before but chances are, the idle sound is only very close to sounding normal. That’s why we need to make some minor final adjustments.

So, you are very close to the middle. A full turn in each direction should make your engine sound out of tune again. That’s why we only take half turns here.

Take half a turn in each direction, try both clockwise and counterclockwise. You need to go back to the middle after each turn. When taking half a turn clockwise, follow with a counterclockwise half-turn.

In which direction the idle sound was closer to normal? You have to this by ear. Let’s say it was closer while turning clockwise. Then, go back to the middle point. This time turn clockwise again but with a 1/4 turn, not a half. Is it better? If not, take a counterclockwise 1/4 turn.

Do you get how this goes? This is how you get that perfect engine sound. Repeat this process until you are satisfied with the engine sound.

Tuning Bike Carbs – Things To Know

People tune carbs for a number of reasons. To get a better power delivery, to get better fuel economy, to fix an engine vibration, or to fix heating issues. So, it goes without saying that doing this takes experience.

Depending on what you want, the process may differ a lot. This guide is all about getting that smooth power delivery, and optimal fuel economy. If you are not here for that, these steps won’t do you any good.

If you have messed with the idle settings screw before reading this, using this guide won’t be helpful. This guide only works if the idle is set on default. If you still want to use this guide, you’ll need to turn it back to default.

What I’m saying is that if you want to increase your engine performance, the normal idle sound won’t do much. If you really want to cut down on fuel usage, again, this is not helpful.

Also, I’m assuming that your bike and engine are in perfect condition. Tune the carb all you want, if your engine is rusted or if oil is leaking, you will still have issues with the engine sound.

Additional Tips

  • Carburetors have default mixture settings. While tuning it’s best to start from these settings. Try finding the product online, look for product details or product info, you will find it there.
  • The idle screw is attached to the throttle, more precisely to the throttle cables. You don’t a screwdriver for the idle mixture screw.
  • Any incorrect adjustments should be counted and corrected before tuning again.
  • If you are tuning the accelerator adjustment screw, warm up by riding not by idling.
  • Some modern bikes don’t have carburetors. These must be fixed with a computer.

Related Questions And Other FAQs

How Do You Tell If A Carburetor Is Rich Or Lean

A carb with a decreased flow of fuel is lean or made leaner. When the fuel is increased it is rich. The easiest way to tell this is to listen to the engine while is idling. If it sounds rough, like it’s coughing it’s lean, if it’s revving, it’s rich.

What Are The Symptoms Of A Lean Fuel Mixture

If your carb has a lean fuel mixture applying full pressure on the throttle won’t be effective. Also, it may sound like you always need to accelerate in order to pick up.

Is It Better To Run Rich Or Lean

Neither, both rich mixtures and lean mixtures are dangerous for your bike. A rich carb can easily overheat, while a lean carb can rust in due time.

Paul Landry
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