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Go Back   Yamaha Forum : Your Yamaha Motor Products Community & Resource > Off Road & Motocross Motorcycle > YZ-F & YZ

YZ-F & YZ YZ250F, YZ450F, YZ250, YZ125, YZ80, etc.

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Old 12-01-2010, 07:38 PM   #1
yz450girl
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Default yz 450 sputter at low throttle

I recently got my hands on a nice 2008 yz450, but I have a question. The guy i bought it from put an after market slip on on the bike and said he had it jetted. I am wondering if it was maybe not jetted properly because at low throttle it will sputter and act like its not getting enough gas. When i first rode it i thought maybe it was where it was cold. (40 degrees) If you get on the gas an open it up a bit the sputter goes away, but at times when u let of the gas it will backfire. What are everyones thoughts? Im thinking maybe it needs to be rejetted? I am new to carbs I usually run FI bikes so I am more accustomed to power commanders as apposed to carbs and jetting. Also would this make it difficult to start?
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Old 12-02-2010, 06:10 AM   #2
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This is a very common issue.
it can be a combination of things
one thing is the air fuel mix screw on the side of the carb, easily adjusted - may help a little. this adjustment is only a small part of the problem usually but can take away some of the pop and sputter. it is easy to do and if done right can be undone easily because of the fact you can count the turns of adjustment and if needed you can just zero it back to where it was . I like to have someone make the adjustment- with help from a friend make an adjustment - take a lap- repeat having a friend play pit crew really helps.

the next thing:
the type of carb with an accelerator pump on it has a leak jet that when adjusted or changed adjusts the amount of fuel that is leaked down or dumped back into the bowl. An adjustable leak jet is around $100 iirc. plus a whole lot of adjustment/jetting changes and it can be made to run much better.
.
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realistically both need to be done to get it to run properly including changes in jetting (pilot - main - needle) possibly leak jet and of course adjustments to the F/A mix screw.

the hard starting issue can be related to jetting and or a sign that you are in need of a valve adjustment.

there is a small amount of bog that is almost impossible to remove from this carb with a 4stroke. most people are unaware of it until they ride a FI version.
your issue sounds like it needs some attention.
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Last edited by macattack; 12-02-2010 at 06:15 AM.
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Old 12-02-2010, 05:59 PM   #3
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thanks. its getting lowered and rejetted this weekend. any suggestions on a jet kit?
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Old 12-02-2010, 07:24 PM   #4
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Sounds like main is ok. Is there an aftermarket air filter to? How much elevation change difference? I'm thinking needle change to smooth it out But at 25 a pop, choose wisely?
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Old 12-02-2010, 08:39 PM   #5
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Its usually a tuning thing involving different size jets - a needle with a different taper . in some cases its easier and better to order the jets from yamaha . I've been told my bike will run better with the 09 jets and needle for example . I'll try to find out if anything like this applies to your year and model and get back with you .
a jet kit is ok if it offers the right jets. realistically you could save a bit of money if you were to get a few pilot jets , a few main jets and maybe an alternative needle. especially if you know the direction the bikes needs to go. as IMG said the pilot jet covers the off idle part of jetting. I cannot say simply changing the pilot will make it run better though.
the person doing the work will probably have to know what jets are in it and hear it run first hand to know what needs to be done.
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Old 12-03-2010, 12:09 AM   #6
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the guy that is doing the work on it went to a tech school to do it. so i have faith in him.. Yeah it actually does have a twin air filter in it. no difference in elevation as the guy i got it from lives 10 minutes from me.
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