yz400f idle speed problem


I already read some topics about that but I still cannot fix my problem.

The problem is:

- no idle speed when engine is hot

- engine stops sometimes when opening the throttle 0 - 1/4 and more slow or fast

What I did:

- cleaned the carburetor and all the jets

- new iridium spark

I moved the position of jet needle from 6 to 4 (standard).

And tried to adjustment pilot screw, I turn in pilot screw until it is lightly seated, and then trying to turn out 1/2 turn and start the engine. Until 3 turns the engine has no idle. I read it should be max 2 turns out, if it is more I need to replace the pilot jet with one size bigger.

When with 3+ turns out of pilot screw the engine runs on high rpms then I need rich fuel, right?

Is it possible without pilot jet replacing, but with jet needle position change to make rich fuel? Maybe that because it was on position 6 from previous owner.

The jet needle does not affect idle jetting. By the way you describe the bike's behavior, you need a richer pilot jet (larger, probably by one size).

The stumble when opening may be cured by that, but if not, then look at the accelerator pump.

I forgot, also cleaned the accelerator pump before all other, because it was not working, was blocked from dirt in the up side. Now the AP also work, but the bike not working better.

I checked my valve clearance

intake 0.10 - a little tight maybe

exhaust 0.20

Is the problem can be from the valve clearance or from TPS?

The valve clearance is fine.

With 4 turns of the pilot screw the bike runs fine.

I will go for larger pilot jet, when I see what size is already installed.

How can I check is everything with TPS is OK?

I got hanging idle, when open the throttle and then close it the rpms getting high, i think that is another sign I need larger pilot jet.

Thats are my current jets

Main Jet 190

Pilot jet 45

Starter Jet 65

Pilot air jet 200

main air jet 100


I am watching this but is there something wrong 20 is like starter jet but standard is jet(#45) STD, mine is 65.

21 is like pilot jet, but standard is JET(#65) STD, and mine is 45.

Something is wrong, I wrong or the scheme?

That illustration has the reference numbers for the starter and pilot jets reversed. The jets listed in the parts list as item 20 (3TJ-14948-**-**) are the pilot jets.

yes grayracer513 they are reversed.

hmm I need to find some other store because I need to pay $38 min only for shipping of one little pilot jet.

Somebody to know some UK, or Europe online store?

grayracer513 thanks for your help in the thread.

I installed larger pilot jet 48 (the old is 45), and there is the same problem, when the engine is warm it doesn't idle and engine stops. Pilot screw 1.5, 2, 2.5, 3

Then I removed the TPS and it looks like the engine idles better, and not stops when rpms are not so low. Pilot screw at 1.5

I think my problem is TPS. How I can measure the TPS, I mean on started engine or not?

Can I ride without TPS? (i know it will have low power, I didn't try to ride it without TPS)

The TPS does not affect the full throttle power output. It's intended to advance the timing at part throttle for improved fuel efficiency while cruising and to improve part throttle "crispness". If you think it's at fault, disconnect it.

Testing procedures are in the manual.

I made a test ride in the mountain,

started from Min Elevation: 238 m (780 ft) up to Max Elevation: 1639 m (5377 ft). In the beginning the bike worked very good, no problems with idle speed, and riding without any problems. But at the high elevation it started to make problems and on idle engine stops, I kick to start it, and it stops after a second.

Why it works good good down and not works up at the mountain?

Maybe I need to set pilot screw in the mountain and not in my garage.

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 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?
    • By bofar
      Hey guys. I could really use your help diagnosing an issue that came up riding today. Bike is a 2013 YZ450F. Well maintained and haven't had any issues until now. Both the oil and coolant are new and levels are to spec. 
      After riding for about an hour in the sand dunes, I noticed a grinding going on on the right side of the bottom end (from what I could tell). When I looked to see what was going on my kickstarter was vibrating and grinding really loud. This was a different sound than if you engaged the kickstarter while running. It sounded less "clanky" and more "vibrating/grinding" if that makes sense. Grabbing the kickstarter and moving it didn't help the issue.
      When I noticed it, I shut the bike off and gave it a second. Being a few miles deep into the dunes, I had to get it out of there so I started it back up and it seemed fine- no grinding. I headed back to the car and after a few minutes, it came back so I shut it off again, started it back up and nothing. That process happened one or two more times before I made it back to the truck. Seemed to run fine otherwise. 
      Once I got it back to the truck I started it up and tried to pay more attention to what was going on but it wasn't doing it. I did seem to have more of a metal on metal noise coming from the motor. I started and stopped the bike a few times and after the motor stopped there was a distinct metal clank/grinding noise that happened right after the motor shut of for maybe half a second. This doesn't happen every time. 
      I tried doing some research and have seen things that make me think it could be the spring that connects to the kickstarter, or the kickstarter stopper plate- but really I have no idea.
      Seems strange that the issue is off and on.
      Thanks for the help!
    • By Goof331
      Does anyone know if there is any differences between the 2 radiators? Will the yz450fx radiator bolt on a yz450f? What I am wanting to do is wire up a fan to my 2016 yz450f. I know how I am going to do it I just don’t want to use a “universal” fan kit. What I would like to do is get a 450fx radiator and the direct bolt on Trail Tech fan for that radiator to have a good clean look.