If you all still remember, I’m still in the process of setting up my Mikuni carb that comes standard with my secondhand 125Z.
Ian Williams here provide a very good details about on how to tune Mikuni carb and I mean really really good details about tuning the carb. Have a read here and I guarantee you will spend your next 2 hours trying to understand what the heck Ian Williams is trying to tell you.

I’m now busy setting up my Yamaha 125Z to get the best out of the engine. I need to change the spark plug since I’m about to tune the carburetor, so I went to the workshop to get the spark plug for the Yamaha 125Z. The shop gave me BP8ES which is the same as the one fitted to my Yamaha RX-Z. Immediately, I opened up my Yamaha 125Z spark plug and stupefied to found out that the spark plug from the engine is BP5ES. Did the shop give me wrong spark plug model? Or is the previous owner use the wrong spark plug for the bike?
To clear things up, I googled the information about it and found that NGK Malaysia provide the best information that I am looking for. So, here it is.
Now, I know the correct NGK spark plug for my Yamaha 125Z is B8ES.
FAQs:
Hmm, what is the difference between BP5ES, BP8ES and B8ES?
-click the link.
Does BP5ES, BP8ES and B8ES fits well to Yamaha 125Z?
-Yes.
So, we can use all those plug to Yamaha 125Z?
-No. Use B8ES.
But, I use BP8ES / BP5ES to my Yamaha 125Z and it is ok and running.
-Yes, you can use it to your bike if you want to. I don’t care.