Swapping the carburetor from a 150cc engine onto a 125cc engine with the expectation of increasing speed is not a recommended approach. While it is technically possible to fit a larger carburetor onto a smaller engine, it is unlikely to provide the desired results and may lead to various issues. Here are a few reasons why:
Engine Compatibility: The carburetor size is designed to match the engine's displacement and airflow requirements. A 150cc carburetor is calibrated specifically for a 150cc engine, including factors such as fuel delivery, air-to-fuel ratio, and throttle response. Using a larger carburetor on a smaller engine may disrupt the air-to-fuel ratio and negatively affect the engine's performance and reliability.
Fuel Economy: A larger carburetor may increase the fuel consumption of your engine, leading to decreased fuel efficiency. This can be especially problematic for a smaller engine that may already have limited fuel capacity.
Tuning Challenges: Swapping carburetors may require rejetting or adjusting the fuel and air mixture, as well as other modifications to optimize the engine's performance. Achieving the correct tuning for optimal performance can be a complex process, often requiring expertise and trial-and-error adjustments.
Engine Damage Risk: Running a smaller engine with a larger carburetor can lead to an excessively rich fuel mixture, which can cause fouled spark plugs, poor combustion, increased wear on engine components, and potential engine damage in the long run.
If you're looking to improve the performance of your 125cc bike, it's generally recommended to explore other modifications that are specifically designed for your engine and have been proven to work effectively. These may include performance exhaust systems, air filters, ignition upgrades, or sprocket changes. It's always a good idea to consult with a knowledgeable mechanic or motorcycle tuning specialist who can provide guidance on suitable modifications for your specific bike model.