FZR600 Genesis - rejetting for K and N's

Published on 12 January 2023 at 18:11
Yamahga FZR600 Genesis side view

In my last post about this bike I had said I needed to rejet the carbs to suit the K and N pod air filters - these things really screw up the air / fuel mixture and modern high revving 4 strokes are really fussy about such things. This one would barely run, it was incredibly lean and action had to be taken. As a rule of thumb there are 3 things you need to adjust on modern constant velocity carbs if you change the airflow, they are; idle air screw, needle height and main jet size. Sometimes you have to go up a size or two on the idle jets but generally speaking you just need to wind the mixture screws out 1  to 2 turns. The main jet is the one that tends to need most attention, it has the most effect at 3/4 to wide open throttle but also impacts on the 1/4 to 3/4 throttle range too. There are dozens of sizes available, the standard jets are 102.5, experience told me to start with 125's, although it's always a lot of messing about to get it just right, as it proved to be in this instance.

I now have it so it will rev right up to the red line but I still think it's a bit lean in the mid range, my gut feel is that the needles need raising, I will have to play some more with it. One thing worth nnoting for anybody doing the same - the fitting of the fake petrol tank seems to make a noticable difference to the airflow at high revs. it seems to richen it up a tad - worth noting to save time, when you think it's about right pop the cover back on and see how you go. Ask me how I know.



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