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Hey guys, I just bought a 04 yfz about two weeks ago and it has been popping and backfiring all the time. It has an R&D flex fuel/air screw and when i turn it all the way in it wont idle but will run mid throttle, so i start turning it back out 1/4 turn at a time and about 2 turns out it starts raising rpm's but the more i go out the higher the rpm's get until the screw falls out. Do i need to bring the main jet down? It has a 175 with aftermarket air filter with the lid off, hot cams, and yoshi pipe. Thanks
 

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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 knob. 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.
Thank you,
Isaac
What about the leak jet?
 
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