WR400 backfiring problem.

8 replies to this topic
  • Thompson

Posted April 20, 2008 - 09:44 PM


Ok my brother has a WR400 and it backfires and the exhaust pipe turns bright red and is hot as h***. so he says the carb needs to be rejetted right so how much does that cost?

  • Michael N

Posted April 20, 2008 - 10:08 PM


It can cost nothing if you find out the settings for your specs and know how to do it.

  • Thompson

Posted April 20, 2008 - 10:30 PM


how do i find that out do u know?

  • Michael N

Posted April 21, 2008 - 11:04 PM


Sorry, i dont. But there are lots of people on this site who will tell you if they happen to spot this thread. Hope you get the info you need.

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  • matt4x4

Posted April 22, 2008 - 03:55 AM


Your brother's bike is lean if backfiring and popping on decelleration.
Sounds like neither of you are versed on Jetting, probably best to do some reading on Carbs and get a basic knowledge of it before digging in, anything we tell you right now will just sound like gibberish to you until you understand how a carb works.
It could also be a clogged pilot circuit, has anything changed on the bike since this started (like - did he add an aftermarket pipe? open up the airbox?)
Anything you can tell us about recent history will help determine things.

  • Thompson

Posted April 22, 2008 - 12:58 PM


no i dont think so, he hasnt added anything to it...plus its been a while sence he even played around with it, he tried rejetting it himself, but that has been a while since he didthat. but i was thinking of bringing it in to the dealer to get it done but i dk how much it cost so i have to call them and ask.

  • matt4x4

Posted April 23, 2008 - 06:35 AM


well, if it was running fine a while ago, now it doesn't and nothing was changed, then it's pretty much guaranteed that the carb got gummed up over time of non use - it's the easiest of all issues to fix - remove, disassemble completelty (except for the TPS - keep it on at all times) throughly clean everything (eddie S. suggests brake cleaner - it's better for the rubber components than carb cleaner). reassemble, set both the air and fuel screws to where they were before (number of turns out), make sure your petcock is working properly too since they sometimes gum up as well.
It should run fine when done

  • bajarider08

Posted April 23, 2008 - 09:48 AM


It is most likely a lean condition. I suggest the JD jet kit, and the AIS smog removal kit. I have an 07 WR450f. It popped and the pipe glowed as you describe. The JD jet kit was $80, and was excellent. The instuctions were perfect. Now my WR runs awesome, no backfire, and no glowing pipe.

  • Thompson

Posted April 23, 2008 - 08:32 PM


how much was the ais smog removal kit? i know he tried changing his jets but i hope i dont have to get new ones prob will...


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