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IIRC (and I'm pretty sure I do), the stock jet is a 125 also.

The kit with new needle worked perfectly in my 79 750B, it don't work worth crap in a 80 KZ750G twin though.

The number 4JN19 is the Mikuni badge number. The Y22 is the badge number Keyster assigned to is clone jet needle. All you need to do is hold them up and compare them. Same length, same taper, etc... they are the same.