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Need Re-Jetting Help? ~YFZ450 Jetting Guide Inside~

350409 Views 646 Replies 253 Participants Last post by  jordanmolinaro98
Our very own "edge" has compiled this great little jetting guide for all of you who are having problems, or need a starting point before you attempt to re-jet your YFZ450's FCR carburettor. Thanks, great! :tup:

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 by roughly 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 the mixture
Stock YFZ with a much-needed re-jet: 168 m 45p #4 clip 2 turns mature
YFZ w/baffle out same as above
YFZ w/baffle out and lid off: #5 or #6 clip same settings as above.
YFZ wipe 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( 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 sure thing across the board but these are VERY close guidelines that you can use to get your YFZ450 to run as it should. I hope this helps get rid of some of the jetting posts that we see every day. 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.

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I have an 07 yfz 450 with a full exhaust, k&n filter with lid off, stage 2 jet kit and stage 1 racing cams. What jets should I run for it?
Hello, I just traded a dirt bike for a 05 YFZ450. It has a HMF full exhaust system with K&N filter. With lid off breaks up around mid range. Guy said he put jet kit in. I am going to check jets this weekend to see what is there. what should be there and what settings should I try ?
it is altitude dependent. but more importantly you will need to read the plug. all of the info you need is in this sticky already.
I have a 2008 yfz450, I currently have a ncvq needle I ordered for the carb. I’m at 900ft above sea level and have an hmf pipe with air box lid on, can anyone enlighten me on the jet/needle settings? And what leak jet should be on these bikes mine has a #90 I believe?
jetting is all about air flow. the most restrictive part of the system is the air box. the fmf may sound better but the improvements in airflow will be minimal at best unless you open that air box. the needle it self does not change the jetting it moves the fuel curve, to match the yz fuel curve if memory serves. moving the needle up and down does fine tune the main jet/fuel mix by bringing more fuel in at a different rpm. given the information you gave i would run factory jets and lift the needle/lower the clip one notch. run it and then check the plug. if the plug looks lean go back to the middle and one up one the main. (i cant remember the factory jetting off the top of my head but rocky mountain atv has a data sheet for every machine). the pilot is probably ok but if its hard to start up one on the pilot. i have never had to change a leak jet on any of the fcr flat slide carbs. jetting is always trial and error and most of what you read on the internet is wrong and will have you over jetted. it really sucks to jet the modern 4 strokes because you have to remove the tank to check the plug but that is the only way to get it right. if you want to get anything out of your current mods you will need to open that air box and put a k&n filter on. there are several options to do that. i cant remember for sure ( i have 6 bikes with that carb) but my yfz with a pipe, jd jet needle and the air box cut 50/170 or 175 is what i run at sea level
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Hey I have a 2006 yfz 450 and was wondering what jetting to use with the air box cover off and a big gun race series slip on, thanks.
So my 2008 yfz is bone stock, so I should go off the showroom guide line right? Stock exhaust with airbox lid still installed? Everyone I talk to say these things run really lean so I don't want to mess it up
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