1998 yz400f carb problem

11 replies to this topic
  • yz400fdude

Posted October 07, 2007 - 05:54 PM


Hey guys I made this little video explaining the problem.
Well at least it runs with the choke on.

Let me know what you all think.

  • jbrooks26

Posted October 07, 2007 - 06:49 PM


Start with the pilot jet, also known as the slow jet. Could also be an air leak, you can test this by spraying some carb cleaner on all of the intake joints. If engine revs when you spray a joint then that is where your leak is. Other options could be idle set too low, try turning it up about 1/2 turn at a time and see if that helps. Try these things and let us know how it comes out. Good luck.


  • yz400fdude

Posted October 07, 2007 - 07:45 PM


I can turn the idle all the way up and it still wont idle but yet I can turn the choke on and turn the idle all the way down and it idles pretty decent. Where is the pilot jet located? There is a stripped screw on my carb and I cant get it all the way apart. I will try the airleak method you suggested also.

  • grayracer513

Posted October 07, 2007 - 08:28 PM


Be sure to check that the hot start is not leaking air by pinching off the hose that leads to the head. If that improves it, it's leaking. Also be sure that your pilot fuel screw hasn't fallen out.

  • yz400fdude

Posted October 07, 2007 - 08:32 PM


Is there an exploded view of my carb somewhere online so I can be sure which jet is the pilot jet?

  • grayracer513

Posted October 07, 2007 - 08:37 PM


Download a maunual:


Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • yz400fdude

Posted October 07, 2007 - 09:05 PM


OK I Looked at the manual so now I know what I did. I already cleaned the pilot jet and all the other jets in the area. So on to looking for air leaks and the diaphragm. Also the pilot screw is there but I do have a question, how tight should it be?

  • grayracer513

Posted October 07, 2007 - 10:16 PM


The pilot fuel screw is he fuel trim adjustment for the idle circuit. As a starting point, it needs to be turned in until it gently seats, then backed out 1 3/4 turns. Then follow the procedure below (it's for a 250F, but it works the same way):


  • yz400fdude

Posted October 16, 2007 - 08:45 PM


Ok guys I've done everything you all suggested and its no better. I'm totally out of ideas.

  • jbrooks26

Posted October 17, 2007 - 05:38 AM


I have one more, take a look at the business end of your idle screw and make sure that it is contacting the throttle cam and actually adjusting the slide position to control the idle. I have seen the cam come out of the TPS slot and this holds it just far enough out of position that the idle screw does not line up with the throttle cam. This will be visible under the plastic cable cover on the right side of the bike. Hope this helps, if you are not sure try to take some pictures and post them up.


  • todds924

Posted October 17, 2007 - 03:22 PM


I'm thinking pilot jet is clogged. Also i have seen FCR carburetors where the slide is put in upside down. On the front of your carb slide, there is a plate with an o-ring gasket. Make sure thats installed proper. Not upside down. Another thing is to make sure your pilot jet and leak jet are the proper size. Just because you can see daylight through them doesn't mean they are clean. You can use numbered jet drills to check. I will post the according jet sizes and the proper number drill to use tomorrow if i can remember my chart.

  • boats203steveb

Posted October 17, 2007 - 04:48 PM


I had a crack in my carb on the out side of the hot start plunger area. mine would also run with the choke out.we even sprayed carb cleaner on the out side of the carb and the idle didnt pick up. so you need to realy look hard if there is a crack.

Related Content


Snake pit oct 30th by The Anvil

Dirt Bike   Dirt Bike Regional Discussion   California
  • Hot  293 replies

Megabomb Fitment by 288yz450

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 1 reply

James Stewart back on a YZ450F by YamaLink

Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  47 replies

Chubby dad, looking at bikes , First trip to the orange/black forum ! by Slow_ride

Dirt Bike   Make / Model Specific   KTM   250-530 EXC/MXC/SXC/XC-W/XCR-W (4-Strokes)
  • Hot  28 replies


Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • Hot  30 replies

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.