The Importance of Carburetor Tuning for Modified Motorcycles

Tuning your motorcycle carburetor is an essential maintenance task to ensure your bike runs smoothly and efficiently. It can also help improve fuel economy and reduce emissions. Some situations may require carburetor tuning, such as after modifying your motorcycle, leaving it idle for too long, or for high-performance activities. However, due to the process’s complexity, it’s best to have a professional mechanic handle it.

a motorcycle Carburetor Tuning

If you want to tune your motorcycle’s carburetor, start by cleaning it with a carburetor cleaner. Then adjust the idle speed by turning the idle speed screw until it reaches the manufacturer’s recommended setting.

The air mixture, the ratio of fuel to the air delivered to the engine, is the next adjustment you need to make. You can find the air mixture screw beside the idle mixture screw on the carburetor. You can use a screwdriver to adjust it until the motorcycle runs smoothly and evenly at idle.

The perfect carburetor adjustment depends on your motorcycle’s make and model, and the owner’s manual usually includes a stock set to follow.

However, if you made modifications to your motorcycle, you may need to adjust the main jet and pilot jet. The main jet provides the majority of fuel during normal operation and top speed, while the pilot jet regulates fuel flow during idle and low throttle conditions. If your motorcycle runs too lean or too rich, you can change the jet sizes. However, finding the correct sizes takes some testing.

How to tune a motorcycle carburetor

Here’s a step-by-step guide on how to tune your motorcycle carburetor:

Clean the Carburetor

  • Remove the carburetor from your motorcycle and disassemble it.
  • Use carburetor cleaner to thoroughly clean the inside of the carburetor, with special attention to the jets and throttle.

Adjust the Idle Speed

Carburetor Tuning for
idle speed
white = idle screw , blue = air screw
  • Locate the idle speed screw, which is usually on the side of the carburetor.
  • Turn the idle speed screw clockwise with a screwdriver to increase the idle speed or counterclockwise to decrease it

  • Adjust the idle speed until it is at the manufacturer’s recommended setting found in the owner’s manual.

adjust the idle speed

Adjust the Carburetor Main Jet and Pilot Jet

The size of the main jet and pilot jet in a carburetor determines the amount of fuel that is delivered to the engine. The size of the jets is determined by their diameter, which is measured in millimeters.

The main jet is responsible for providing fuel to the engine at higher RPMs or when the throttle is opened wide. The pilot jet, on the other hand, is responsible for providing fuel to the engine at idle and low RPMs.

To adjust the jet sizes, you may need to replace the stock jets with larger or smaller ones depending on your specific needs. This is typically done by removing the carburetor and accessing the jets from the bottom.

For example, for a Honda CRF150R, the stock main jet size is 135, and the pilot jet size is 40. However, it’s important to note that jet sizes may vary depending on the year and model of the motorcycle, as well as any modifications that have been made to the engine or exhaust system.

When setting the main and pilot jets, it’s important to pay attention to the engine’s performance and adjust the jets accordingly. A rich fuel mixture will cause the engine to run too rich, resulting in poor fuel economy and reduced power. On the other hand, a lean fuel mixture can cause the engine to overheat and suffer from premature wear and tear.

Remove the spark plug after running with the adjustment.

  • If the plug color is black, the screw does not have enough turns and needs more to get the lean setting.
  • If the plug color is white, the motorcycle is running too lean, and you need to increase the main jet size.
  • Adjust the main jet and the pilot jet if necessary to fine-tune the fuel delivery for your engine’s specific needs.
spark plug colors
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Test Ride


  • After adjusting the carburetor, take your motorcycle for a test ride to see how it performs.
  • Observe how it runs at different speeds and RPMs to determine if any further adjustments are needed.

In conclusion, tuning your motorcycle carburetor ensures optimal fuel-to-air ratio for engine performance, fuel economy, and emissions. Factors like altitude, temperature, and humidity affect the need for carburetor tuning. If you experience problems with your carburetor or seek high-performance activities, you may need to tune your carburetor.