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Our very own "edgerat" 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 carburetor. Thanks edgerat! :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 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.


 

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I would like to ask one quick question before I do this!
As of right now I just have my baffle out with the stock jetting, if I take the airbox lid off and rejet to your specs will I be able to tell a difference in power? Also since I am taking the airbox lid off it would be a good idea to buy and aftermarket airfilter kit right? Thanks! :tup:

Oh and sorry to be a pain, but I just read on a site where I am ordering my nerfs from. They sell pilot jets and main jets for different atvs. When I rejet does that mean that I have to buy all new jets? Sorry i am dumb with carby work. :nod:
 

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You will be able to tell a HUGE difference when you pull the airbox lid off and re-jet. You are dumping a lot more fuel and air into the engine so it pulls WAY harder. It is a good idea to go to a higher quality air filter as soon as you can afford it. The stock one is fine but a good K&N or TwinAir filter is 100 times better than stock. Yes you will need to buy new jets in order to re-jet. www.jdjetting.com or go to your local yamaha dealer and buy a GYTR jet kit for the YFZ450. They are both in the 60 dollar neighborhood but you are purchasing something that will help your quad work better for years to come.
Isaac
 

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Alright then, thanks. And as for the gytr kit that will be in about the $120 neighborhood, I live in Australia! But I have the money right now so I will just order my nerfs and the gytr kit all at the same time. :tup:
 

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So is the 180 main hmf recommended too big?? I have a 180 main and 45 pilot right now, it seems to run alright but there's a little stumble off of idle (which I thought the pilot would fix). Also, I am shooting fire out of my pipe, is this caused from a rich condition or simply because i don't have a spark arrestor? Thanks for the help in advance
 

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backfires are because of either too much air or too much fuel. Mine throws 4 foot flames when it is cold. the 180 that HMF recommends is not wrong but it will work if you dont want to mess with anything else. If you take the manufacturers recommendations you will be very close because different tuners jet differently. The jetting specs that I gave are very generic and work well for the Baldwin, HMF, Sparks GYTR pipes that are out there now. I talked to TC today and his pipe requires WAY different jetting.
Isaac
 

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here is jetting issue for those of you in the know
...

stock pipe with sparky removed
stock air box with lid ON and GYTR dual stage foam air filter
GYTR needle
1000 foot elevation

main 165 ?
pilot 45 ?
clip #3 on GYTR needle?


HELP, I'm an ameteur with 4 strokes.. but much better with 2 strokes
 

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sounds about right to me. Do you have any hiccuping or hesitation in the power? Try warming the quad up and then slowly increasing throttle to about half throttle and then blipping it wide-open for just a sec. Did you notice any roughness in there? Where was it at in the throttle travel? All of these things will affect the jetting. Try and see what it does with the jetting you mentioned.
Isaac
 

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Please help!

I've got a YFZ with fmf titanium 4, power bomb & YZ cam im getting a bit of hesitation from idle example if im at dle and open to full throttle it kinda shuts of the n sometimes pics up. what jets should i be useing and can I use the YZ dirtbike jets?

my guess was 168 mains 45 pilot and clip pos 5-6 running with lid on! as said above but just wont to be sure as it is an fmf pipe
 

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You can use any jets that are for the Keihin FCR carb. Also the off idle hesitation is caused by an off setting in the mixture screw. I altered the setting on mine and was able to get rid of the hesitation. Try 2.5 turns out on the mixture screw and see if that solves your problem if that doesnt work keep messing with it until it runs to your satisfaction.
Isaac
 

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thanks for the advice but to be sure i play with the right screw where is it :oops:
 

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forget he last post i found it :hey: my hesitation hasn't improved and have noticed it idling and very low revs are a litle rougher but my deacceleration popping has stoped and it kinda seems more punchy :tup: the pilot thats in is a 42 so should i try a 45
 

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yeh i really does help, I've also found that these carbs are fairley easy to work on as each day goes by i'm finding more and more good things about the yfz 450 :tup: I'll get the 45 pilot can i ride until they come in i don't think my dealer will have them for a few days friday is when i should get them
 

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I got the TC pipe yesterday and his recommendations are whacked. Stock main and pilot, clip in 3rd position mixture 2 1/2 turns qith lid on stock cam timing. Well my but cause I got a 5 inch flame threatening my back number plate. I'm gonna try the 168 and 45 tonite and I will post my results, oh and there is no way that lid is staying on either :hey:
 

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Discussion Starter #16
I just added a picture of the main, pilot, and starter jets in the original post above since some are confused about which is what.
 

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How do you know if you're jetted too rich on the main jet? Can you put a tachomoter on the YFZ to see if jet changes affect the wide open throttle RPM?

BTW- do inductive tachometers work onthis engine since the secondary coil is connected right to the spark plug?
 

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RG, I would take Tom's advice. He knows what he is doing. I talked him about a week ago about getting one of his pipes and he told me the same thing. Of course he also thinks the cam mod is BS so I dont really know what to thing of that.
RPYFZ,
You can tell if you are too rich on the main if you cant get to the rev limiter. But as a general rule the richest you would ever need to get is a 170 main. I know that HMF calls for a 180 but that is if you dont change anything else.
Isaac
 
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