Have you ever thought how will modifying the jets (main jet, pilot jet etc) in your carburetor can improve its performance?
Let me explain in most simple words.
Okay, now look at the picture. As you can see, from 0-1/4 opening of the throttle, the pilot jet takes the job to deliver fuel to mix with air inside the carburetor.
After the 0 opening and before it reach 1/3 opening, throttle valve takes the job.
After that, from 1/6-3/4 opening, needle jet takes the job.
Next, from 1/4- <3/4, jet needle takes the job.
And finally, the main jet takes the job after full opening of the throttle valve.

Then, how to tune this carburetor?
The basic rule of tuning carburetor is to make the fuel-air mixture remains consistent at any throttle valve opening, but you can also tune it to your preference.
For example, let say I have my bike to have good mid throttle valve opening performance for better pick-up for mid-RPM. So, what I can do is to modify the jet needle only. I need to get a thinner jet needle so that more fuel can be mixed when throttle valve is in 1/4-3/4 position.

One more thing, when you bore the carburetor to get a larger diameter for the airflow, you also need to modify or replace all the jets to a suitable one. You need to get thinner jet needle, larger diameter needle jet, larger main jet and larger pilot jet. If not, the mixture of the fuel-air will become to lean (less fuel, too many air).

Simply said, the bore size of the carburetor air intake must be proportional with the size of the jets.

Now, you can try to modify your own carburetor. Do this with your own risk!

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