YZ400F Won't idle and Backfires

9 replies to this topic
  • bblade

Posted December 22, 2009 - 02:50 PM


Only with the choke on it starts up after a few kicks but it wont idle and it backfires a bit. After running for about 20 seconds it just dies like the kill switch was pushed. I dont understand why it's not running because I just gave the carb a good cleaning and replaced all the gaskets and O'rings. I even rebuilt the accelerator pump. All the jets are stock sizes, and the needle jet is set to the stock number 4 postition. The piolet screw is set at 2 turns out, but I have messed with different positions several times to try and get it to idle correctly but nothing works. It's getting good spark so I can rule out that part. The air filter is clean and in good shape. I made sure all my conections were tight so I dont think it's a air leak issue. My next move is draining out the gas and putting in new, but other than that can anyone help me????:moon:

  • theunclebeef

Posted December 22, 2009 - 02:59 PM


Did you connect the carb to the air box hose? Its behind the frame just out of eye site. OR stuck float?

  • bblade

Posted December 22, 2009 - 03:09 PM


Yes, I made sure all the hoses were conected. And I adjusted the float level when I put the carb back together. Would not having the seat on over the air box have anything to do with this? Maybe it's getting to much air or somthing.

  • theunclebeef

Posted December 22, 2009 - 03:35 PM


Sounds like your starving the engine? Maybe the acc pump? But Lots of pops is usually an air leak!?

  • theunclebeef

Posted December 22, 2009 - 03:37 PM


E-mail this guy:

Eddie Sisneros
TT Intake Expert
Visit the Intake - Jetting / Fuel Injection Forum

TT Main home page.

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  • bblade

Posted December 22, 2009 - 03:53 PM


Do you think the TPS has anything to do with the problem? Or maybe the needle jet needs to be replaced because it looks like it could be worn just a bit at the tapered end. I forgot to mention when it backfires, a little flame comes out the tailpipe. I always thought that ment a rich mixture.

  • theunclebeef

Posted December 22, 2009 - 04:07 PM


My 98 400 backfired like crazy, too lean, I changed the jetting and solved that problem. Backfireing was kinda was normal for those bikes. Get it to run then worry about the backfireing. Did you change the needle clip position?

  • Jaspers Yz

Posted December 22, 2009 - 04:30 PM


I just had an air leak on the intake boot on my 99 400. Got a new one the old rubber just had it and was old. Once there was a little heat in the boot it sealed better but I sprayed a little either or cleaner next to the boot and the idle would pick up just enough to tell me that there was a air leak

Edited by Jaspers Yz, December 22, 2009 - 05:12 PM.

  • lumpy790

Posted December 22, 2009 - 04:49 PM


There is a strange O ring between the float bowl top and the slide body that I had get dislodged and partially blocked the passage ways. To get to it you need to remove the torx screws.

  • bblade

Posted December 22, 2009 - 07:36 PM


I have not changed the needle jet position yet. What hight should I change it to? What pilot jet would you recomend changing to if that was your next step? It has a 45 installed right now. Maybe one size larger? And what about that TPS unit? Does the position of it really make a difference when it comes to running at idle? I thought it only works when you twist the throttle. My biggest concern is to just get the thing to idle. I can fine tune it from there...

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