Simple Steps To Adjust The Carburetor Air-fuel Mixture On Motorcycles

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In any motorcycle, the carburetor and engine work in tandem to provide the ideal revolutions per minute. However, in many cases, even with the highest quality motorcycle chain, carb, engine, etc., the ideal RPM is not achieved. 

It happens because the concentrations of fuel and air circulating between the carburetor and the engine are not optimum. Moreover, the effect of the air-fuel mixture is not just limited to the RPM. 

In the long run, using the motorcycle with an incorrect air-fuel ratio can lead to engine damage. Thus, in this article, we shall tell you how to adjust the carburetor air-fuel mixture on motorcycles and how to check the fuel levels.

How To Identify Whether The Ratio Is Incorrect? 

If your motorcycle engine is not working properly, it can be because the ratio of air and fuel concentrations is not correct. Either the concentration of the fuel is high, or there is too much air entering the carburetor. 

When the fuel is in excess, it is called a rich mixture. Similarly, when there is too much air, it is called a lean mixture.

Signs Of An Excessively Rich Mixture

The engine and the motorcycle behave differently with rich and lean mixtures, respectively. Generally, when the motorcycle has a rich fuel-air mixture, it shows signs such as dark and heavy fumes. The engine starts producing muffled noises, and the performance of the motorcycle declines when the engine becomes hot.

The throttle is unable to rev up the engine, and it is difficult to obtain the desired acceleration. Moreover, the engine performance improves on removing the air cleaner. 

However, the performance declines significantly if the choke is on. If you continue to use the motorcycle in this state, parts of the motorcycle, like the spark plugs, start wearing out because of excess soot deposition. 

Signs Of An Excessively Lean Mixture

Similarly, using the motorcycle with a lean fuel-air mixture has several adverse effects.  Generally, with such mixtures, the engine does not respond, even when you pull the throttle forcefully. Moreover, the engine tends to get heated after a short while. 

It results in poor acceleration of the bike. In the long run, the engine health deteriorates, and parameters like mileage and revolutions per minute start decreasing.

Popping sounds are heard in the carburetor and the exhaust when you pull the throttle. The overall bike performance decreases, and the engine works well only in warm conditions. Under warm conditions also, the engine only responds when either the choke is off, the throttle is closed, or the air cleaner is removed. 

Ambient Conditions And The Fuel-Air Mixture

Although both types of mixtures can affect the motorcycle performance adversely, the kind of mixture that your motorcycle requires also depends on the ambient conditions.

For instance, at higher altitudes, with high air temperatures and high humidity, a lean mixture is most suitable for the motorcycle. 

Similarly, for a low humidity level and low air temperature, a rich mixture works best. 

Mechanism For Maintaining An Ideal Mixture

The carburetor floats are responsible for maintaining fuel levels in the oil reservoirs. The fuel that enters the engine from the carburetor passes through standard orifices. These orifices help in fuel metering. 

The fuel is propelled out of these holes by the inlet of air into the carburetor. It creates a pressure difference to push out the fuel. However, the float system closes the air inlet when the fuel levels in the reservoir reach an ideal value. 

The float system has a carburetor float needle, attached to a floating screw. As the name suggests, the needle floats inside the motorcycle oil and helps in indicating the fuel levels in the carburetor bowl. 

Generally, the motorcycle manual prescribes the ideal amount of fuel that the float needle should indicate. 

Checking Fuel Levels

With changing ambient conditions, you need to let in some extra air into the carburetor bowl. For example, in higher altitudes where the air is thinner, you need more air inside the carburetor to balance the mixture. Thus, in these conditions, you need to check the fuel levels manually. 

Experts suggest that the best way to measure fuel in the carburetor bowl is through the clear tube method. 

Follow these steps to check the fuel level with the clear tube method:

  1. You will require a clear hose for this process. Take the clear hose, and connect it to the drain hole of the carburetor bowl. Generally, a lot of grease accumulates on the drain hole of the carburetor. Therefore, ensure that you clean the hole before testing the fuel level. 
  2. After ensuring that the carbs are level, start loosening the drain screw of the carburetor bowl. 
  3. Now hold the clear hose upright against the carburetor bowl and check the fuel level. 

Adjusting The Fuel-Air Mixture

Before we describe the steps for changing the air-fuel concentration of the bike, you need to know about the idle screw. The idle screw regulates the revolutions per minute, and it is linked closely to the throttle. It is not fixed firmly. Therefore, you do not require a screwdriver to tune it. 

Steps For Adjusting The Fuel-Air Concentrations

  1. To start the process, tune the idle screw so that you get a high rpm.
  2. After this, start tuning the floating screw. When you move the floating screw clockwise, you get a lean mixture, and on moving it anti-clockwise, you get a rich mixture. 
  3. Initially, with higher revolutions per minute, start turning the floating screw clockwise to make the mixture as lean as possible. You will notice that the revolutions start decreasing. Continue until you get the leanest mixture possible. 
  4. Now gradually tune the floating screw anti-clockwise, making only ⅛th of a turn at a time. When you do this three to four times, you will see that the revolutions first increase and then become constant. 
  5. When the revolutions become constant, stop tuning the floating screw. It is the optimum setting of the floating screw, allowing the ideal amount of air to enter the carburetor. 


So, in this article, we taught you how to adjust carburetor air-fuel mixture on motorcycles. As you can see, maintaining a proper air-fuel balance in the motorcycle is important for good engine health. While it can be adjusted by tuning the floating screw, turning the screw and identifying when the revolutions become constant is something that you can only learn with practice. 

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Nathan Olvera

Nathan Olvera

Nathan was born and raised in Nevada where he spent some of his earliest days cycling around the neighborhood and gradually developed a great deal of love for motorbikes.
Nathan Olvera

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