How to adjust carburetor air-fuel mixture on a motorcycle

To maximize your performance on your motorcycle, you need to know how to adjust the carburetor air-fuel mixture. 

If you are not happy with the way your carburetor is performing, the chances are you need to adjust the air/fuel mixture. Changing the carburetor air-fuel mixture is the best way to get the best performance out of your motorcycle. Before you continue reading, please do not attempt to modify your motorcycle’s carburetor air-fuel mixture unless you have gotten sufficient training from a professional mechanic.

Prepare Your Engine

The saying goes, a cold engine runs differently than a warm one. It’s essential to adjust it to get the best performance from your engine. Here are some pointers to help you achieve just that.

Start your motorcycle and let it run for at least 5 minutes. While your motorcycle is idling, you should be listening to the sounds of the engine and the air-fuel mixture. This will help you determine if the air-fuel mixture is correct. You’ll be able to tell if your bike is running too lean or too rich by hearing how it sounds.

When revving your engine, make sure no one is around. You may be revving your engine quite a lot so that it makes some noise. If you live next to any neighbors, they may complain about it. It’s better to do it in a safe place where no one can see you.

Find the mixture screw

The first step is to find the carburetor. If you’re not familiar with it, find the air filter, and you’ll find the carb. The air filter is easy to spot. On motorcycles, it’s facing the rear wheel. 

Then, find the screws. You will normally find three:

  • Air Screw:  Controls the air mixture.
  • Fuel Screw: Controls the fuel mixture.
  • Idle speed screw: Controls the idle mixture.  

Please note – the location of the screw will depend on the kind of engine you have; they are typically placed side by side. The idle screw, however, is on top of the carb, away from the engine. 

Adjust to a Leaner Mix:

Grab a screwdriver and start adjusting. I cannot stress how complicated this process so be very careful. 

Tighten the fuel mixture by turning the fuel mixture screw clockwise;  this will make it leaner because it will reduce the amount of fuel getting it. When the engine starts sounding rough, turn the screw clockwise.

A lean mixture is a mixture that contains less fuel than the amount required to burn it. When your bike starts to idle at a lower RPM, it means your engine is burning less fuel and therefore it is not producing enough power.

Adjust to a Richer Mix:

A lean mixture is a mixture of gas and air, which is not ideal for an internal combustion engine. A rough-sounding engine indicates that there is insufficient gasoline in the mix, as well as excessive friction between parts, which leads to damage.

You’re going to loosen the mixture by hand. Keep turning the screwdriver counter-clockwise until you hear the engine start sounding like it’s revving. When you’re ready, stop turning and begin counting the number of turns you’ve made. The goal is to get to a ratio between these two extremes, but you can also do this by ear.

Find the Middle

I mentioned earlier that you should count the number of counter-clockwise turns. I hope you did. You need to find the idle that your motorcycle used to have. If your start and fuel usage is off, then something is wrong. The solution is to find the middle ground between a rough and revving engine. 

Find the sweet spot by turning the screw clockwise again. For example, if you made three full clockwise turns, you need to make 1.5 counter-clockwise turns. If you’re successful, your motorcycle’s idling should return to normal.


While turning the screw, you may have already reached the middle spot, but you couldn’t tell because it sounded close to normal. Hence, the need for fine-tuning. 

When fine-tuning, only make half turns. A complete turn in either direction will make your engine sound out of tune again. 

Make a half turn in each direction, then return to the middle. Listen to the sound of the engine. Which direction does the idle sound closer to normal? Does it sound normal when you turn counter or clockwise? Then, from the middle, make another turn, counter or clockwise, this time, only 1/4. Which sounds better? 

Once convinced that the engine sounds better, you can repeat this process. 

Tuning your Motorcycle’s Carburetor

There are various reasons why you should tune your motorcycle’s carburetors.:

  • Fuel-efficiency
  • Better power delivery
  • Eliminate engine vibration
  • Solve heating issues

How to Fix Incorrect Adjustments

If your motorcycle is poorly maintained, more often than not, it will need adjustments. The constant vibration of the engine causes the screws on the carburetor to self-adjust and this often needs adjustment. You have two alternatives at this point: make the modification yourself or take it to a technician.

Do you know if a Carburetor is Lean or Rich?

 A carb with a low fuel flow will be “lean.” When you increase the fuel, it is “rich;” this can be heard in an engine when it’s idling. It sounds rough or rough-sounding when it’s “lean.” 

Full throttle doesn’t always translate to the best performance. It may not be the most effective if you apply pressure at full throttle. Acceleration may also feel like you need to work the gas harder than you should.

As you go through modifying the fuel system on your motorcycle, it’s important to remember that you need to hit the right spot between rich and lean to keep everything running safely. Rich mixtures and lean mixtures are bad for your bike. If you don’t maintain a proper air/fuel ratio, you can damage the fuel delivery system in the end.


Your motorcycle’s engine is a big piece of machinery. If it isn’t running properly, it can lead to many problems. It’s important to maintain your motorcycle to perform at its best. If you’re having any trouble with your bike, you should bring it to a mechanic. It’s far less expensive than attempting to repair it yourself.