2008 YZ450F will not idle

Here is my dilemma. I have a stock 08 YZ 450f and it will not idle or stay running unless the throttle is given gas. I have messed with the idle screw and fuel, but this does not help to set the proper idle for the bike. It seems like the idle screw does nothing to help when I turn it or out. However the fuel screw does change the running of the bike, but not the idle. The fuel screw setting is currently set at 2 3/8.

I have reviewed the below link to make sure my jetting spec are correct, which they are. I even purchased the R & D Fuel per the August 08 MXA Mag and this has not helped. The carburetor has been ripped apart a number of times and cleaned to insure that nothing is clogging the jet’s etc. The gas in the tank has been drained and replace as well.

2008-09 YZ450F User Jetting Database


Can anyone help me out?


throttle cable?

What about the throttle cable?

is the hot start plunger stuck?

Nope...the hot start plunger is stuck, I can move in or out.

It sounds like a classic clogged pilot jet to me, Makes sure the jet AND the passage are clear, There is something restricting the pilot circuit.


Like I said I have cleaned the carb, but will rip it apart again to make sure that nothing is restricting the jet and passage..


The idle speed screw, specifically the threaded end of it, must contact the throttle linkage to hold it open. As obvious as that probably is, you might want to remove the linkage cover from the carb and have a look.

Ok, I clean all the jets in the carb again, blew it out with air and put it back together. The first couple of kicks produced the same results. The cleaning of the jets was good test to insure that they were not clogged. I then turned in idle speed screw kicked the bike over an it started right up with a nice idle speed. I did however have to mess with the fuel screw since the temperature around 28 degrees here in NJ. Took the bike for a test ride and it ran awesome!! I would like to say thank you to everyone for their help and input to help me resolve my issue. This site is great!!



Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

  • Similar Content

    • By Woody Oakson
      So I have a 2008 yz450f and I would like to add some LED lights to it to make it street legal. Mainly just a headlight and a tail light and turn signals if I need to (which where I live hand signals work).
      So where I need help is getting power from the stator to a battery. Wiring from a battery to anything else I got and understand and don't need help with (but recommendations are welcome). So my questions are.
      What type of power is coming from the stator. AC or DC? Will I need a regulator or rectifier? How do I wire something from the stator to charge a battery? What would that wiring look like? Would this stator http://www.rickystator.com/product/stators/yamaha/yamaha-yz450f-35-watt-lighting-stator-assembly-06-09 work? Anything else I should know? Any help would be appreciated. Thank you.
    • By Samx22x
      Alright so I’m new to the site so maybe this question has already been asked. I have a 2002 Yz426f and I want a new exhaust, it has the stock header and silencer and sounds amazing but I’d like a little more from it, I know I want the fmf power bomb header and am kind of undecided on the silencer, I see on fmf’s website that their silencer will add low end and slightly take from top end and the other way around but I’m looking for increases throughout rpm range. I do a lot of trail, hill climbs, jumps, and wheelies so kind of need good performance everywhere in the rpms. My question is what silencer I should get and the jetting I should get for the carb. Thanks
    • By Zack Ailing
      hey guys just picked up a new project bike. Its a 2008 Yz450f with a big bore kit and a stroker crank. The total cc of the engine is now 499. It runs on race gas and will be an absolute monster when shes ready to rip. I will continue to make more videos on the bike so stay tuned Link to the video: 
    • By High On Octane
      2005 YZ450F, Athena Big Bore, Power Bomb, Q4.
      Ok.  Last fall after a decently hot day at the track, I noticed coolant leaking from the weep hole on my water pump after pulling into the pits.  Ordered an Athena re-seal kit, put it all back together and didn't really get a chance to ride it.  3 weeks ago, went out to the track, and after about 20 laps, it started leaking again.  Kept filling it with water to get thru the day, never ran anywhere close to dry or empty, jut a cup or 2 low.  Ordered a Hot Rods re-seal kit with bearing and also ordered a new w/p shaft.  Did it all again and got it back together, ran it, and as soon as it got to temp, it was leaking out of the weep hole again.
      So, what the heck am I missing?  I'm totally stumped on this one.  Is something wrong with my case cover?  Do I need to replace the w/p itself?  I'm feeling like I should've just ponied up for the Boysen in the first place, and now it's biting me in the butt.  Any help or suggestions is greatly appreciated.  Thanks!
    • By Awheeler25u
      Alright guys, my 04 YZ450f runs fine minus some slight backfires when decelerating (need to adjust fuel screw). The other day when I was done checking my valve clearances I changed the oil and filter and put the bike back on the stand, kicked it a few times without the ignition coil in to get some oil flowing got off and noticed a little drip spot on the ground, it was no bigger than a dime so I thought nothing of it. Started the bike with no issues took it for a quick 10-15 min ride, came back to the garage to find out my whole case was soaked with oil and it started pooling in the low spots on the clutch basket side. I cleaned all the oil off and took it out again for 5 mins to try and see where it was coming from, here are my three possible places but nothing directly leads me to believe it’s any of the three either.
      1. Decompression plug area
      2.  Valve cover gasket leak (not really sure because the amount of oil leaking Looks to be a large amount
      3. Oil tank (bad weld or cracked weld somewhere
      what are you guys thinking?