08 WR450F Starting issue - one sputter & backfire, dead

6 replies to this topic
  • theo.sti

Posted September 25, 2012 - 07:50 PM


Hi all, i'm new to the forum, a quad bike converted dirt rider now with my first used 2008 WR450F. The prev owner looked well after the bike but didnt do any free mods and as i couldnt ride the bike after i bought it due to a busted knee figured id do all the free mods: ais removal, grey wire mod, snorkel out, o-ring mod, etc bike's got an fmf ti on.
The bike always fired first kick or swing, but after going on dirt trail it started sputtering and backfiring and wouldnt go again. i pulled the carb at home and found a 175 main jet ( i thought too big but bike ran well) cleaned carby and put back in.
The bike fires once, sputters, then backfires and dies. i pulled the carb many times now to make sure i didnt miss any dirt, i dropped the fuel, changed to the suggested 168 main jet for my area, still no starty.
So yesterday i filled all day and changed the plug back to the old one i replaced and the bike started first thing, ran beautifully, took it around the block, dies half way so me pushy back. Change out plug again to new one, what do you know, it works again. So at this point i believe my problem is on the actual plug that clips onto the coil pack that sits on the plug as it works fine every time i WDF it.

Has anyone heard of this, does it make sence? I must add that on the dirt trail the the problem increased every time i went trough water untill the bike eventually died.

I have now degreased the plug and wires, dried it and siliconed it up nicely, hopefully IF my diagnosis is correct my prob is solved, but i would love some forum guru opinions.

  • MANIAC998

Posted September 26, 2012 - 03:38 AM


Did you write down what jetting you found while inside the carb? If you let us know what jetting your running, we can be a little more accurate on our diagnosis. From the sounds of it, either your jetting is off and is fouling your plug, or something is-a-mis on your ignition which is fouling out your plug. I would think the jetting is more than likely the cause.

  • mongial

Posted September 26, 2012 - 05:01 AM


I had a similar issue (similar, not exactly) on an 09 WR 450 and found a frayed ground wire right off the battery. Two wires invovled, one going to the ignition and the other I believe to the rectifier/ regulator. Fixed the fray and has ran fine ever since. Good luck and I hope its an easy fix.

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  • theo.sti

Posted September 26, 2012 - 07:47 PM


Maniac, the jetting I can only remember the #38 leak off, not the rest, but I can assure you its not jetting, the bike runs perfect and super strong when it does run.

Update... Yesterday I thought all was sorted after the silicone job, but no, bike runs and idles, but on throttle dies.
Bike starts very easy then as soon as I touch and turn or wiggle the coil pack on the plug it dies. I got it running reliably by pushing the coil pack in as hard as i can, I just hope it stays there, today ill find out what replacement cost is or if I can try one on appro, to see if it solves my problem.

I dont think the coil pack is nacked as it should stop working consistently as it heats up no? but never the less, my situation seems like a new thing or not heard of on a WR, at least now its documented.

Mongial, I checked my wiring directly off the bat, all seems good, but i will keep that in mind.

  • MANIAC998

Posted September 27, 2012 - 01:43 AM


Does the coil pack on top of the plug look damaged in any way? What exactly did you have to silicone? Pics would be nice, so we can see what your seeing.

  • theo.sti

Posted September 27, 2012 - 08:44 AM


You are looking at the plug that clips onto the coil pack.
I think I had 2 separate issues one was possibly the 2 wire in the plug in the pic that was shorting out and the other like I mentioned is the coil pack that wasnt sitting tight enough.

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  • mtahoe

Posted October 01, 2012 - 10:36 AM


Sounds like what I went thrrough. Same thing was going on changed the spark plug coil and all good bet thats it mine was back firing/ stalling and very intermitent.....good luck went through everything not the carb back to stock jets runs perfect 6-10k...... starts first kick again :)


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