I have heard many people say that stock YFZ's are jetted very lean from the factory. My 05 is bone stock with the exception of the baffle/silencer being removed, It has never been jetted before, I was just wondering would it be worth it to re-jet, I live in Western Massachusetts, hot summers, cold winters I would like to have Jetting that I can use all year round, will it give me more power?, is it worth it? I dont plan on modding it anytime soon... If so, what sizes should I go with?? Thanks for the help guys.

 

If you've got the baffle out you are probably a touch lean already.

165 or 168 main, 45 pilot, and you should be good for just about any conditions. If you want to optimize then yuou'll need to change for weather.

 

The following is for 04-05 yrs

This is a pretty basic outline for jetting on the YFZ450. These settings are for sea level if you are anywhere from 1500ft to sea level these settings should be very close to right on. Anything above 1500ft should lean the settings accordingly. Also, as a good adjustment you should raise the idle rougly 10 steps on the ****. The dealers set the idle way too low and that is a big cause for stalling these things.

Stock YFZ off the showroom floor: 158 main 42 pilot #3 clip on needle 2 turns mixture
Stock YFZ with a much needed re-jet: 168 m 45p #4 clip 2 turns mciture
YFZ w/baffle out: same as above
YFZ w/baffle out and lid off: #5 or #6 clip same settings as above.
YFZ wpipe and lid on: 168m 45p #5 or #6 clip should be very close

Now, for this next set of jetting you will need to buy either a James Dean jet kit(www.jdjetting.com) or a GYTR jetting kit from your local dealer.

YFZ w/pipe and lid off: GYTR needle #4 clip 168m 45p 2.5 turns mixture screw.
YFZ w/pipe and lid off: JD blue needle #3 168m 45p 2.5 turns mixture screw.

The above settings are somewhat rich but this engine craves tons of fuel and tons of air to make big HP. I have yet to foul a plug running the JD Jetting settings on my YFZ with a Sparks pipe and the YZ exhaust cam. Also all the pipes out for the YFZ are going to be pretty close to the same jetting-wise so this FAQ should get you pretty close. Again, these settings cannot be a for sure thing across the board but these are VERY close guidelines that you can use to get your YFZ450 to run like it should. I hope this helps get rid of some of the jetting posts that we see everyday. If you have any questions about how to alter your jetting to dial it in exactly please ask the questions here so that we can all learn from your testing.

 

Hey thanks alot for the help, I really appreciate it. Im prolly gonna jet it sometime this weekend so I will post my results on here so people can learn from me, thanks again.

 

Stormnorm is right in there. His advice would be an excellent starting place. May be spot on perfect for you.

If piped with no lid.....
Try a touch richer in cold weather for 04-05 models.
Perfect for 06-07 models in cold weather too. But lean it up a little (160-165 main) for 06-07s in warm weather.

06-07s like to be about a step leaner than 04-05s. Not sure why (displacement? Timing?) but Yamaha recommends it on thier gytr jet kit and I found it to be right on on my 07.

YFZ's are a good thing, no?
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