wr 426 backfiring

just wondering if anyone has had any problems with thier bike back firing and fouling plug ?

my mate was told it was proberly because the exhaust was so restricted from stock so he put an fmf power core rear muffler on it, and removed the airbox top cover, checked the jetting on the carby and it was all stocked, checked timing all ok, he replaced the plug and ran fine but after a couple of rides and letting it sit for a week or two it would be hard to start again and run rough with the throttle open backfiring untill u rev the shit out of it and ride hard for a few minutes then its fine for the day, till next ride.

when it's backfiring he replaced the plug straight away and bike starts first kick and doesnt miss a beat till you let it sit for two weeks and after a couple of rides.

He has put a iridium plug in his bike but it still does it, if any one could help it would be great thanks

I would do a valve setting check before you do anymore rides and also check that the exhaust seals are still in good shape. If any of the exhaust gaskets are worn they will allow air to get into the muffler and cause backfiring. As for the hard starts that has typically been valves out of adjustment or I found on my '02 WR426 that every other year I should replace the jet needle and the needle jet, they seem to wear on each other. While your in the carb, check the O-ring on the float needle seat(you have to remove it to check). Hope this helps -- WR Dave

That’s good advice from Dave. Start with the basics and go from there. Backfire is usually a lean mix or an exhaust leak while a fouled plug points to a rich mixture so it seems you are dealing with more than one issue.

Thanks wr dave and byggd i will get them checked I know that the exhaust valve clearance are good but the intake has a little bit much clearance would that cause the backfiring


i have almost the same problem with my 426 -02.

i have yoshimura full system and removed removed the airbox-top, the jetting is stock and my bike runs great for 30minuits and after that its backfiring when i´m in gear and accelerating but no problems when idiling or reving in neutral. if i replace the plug it runs great for 30min and then its the same problem again. when i replace the plug it is black.

any ideas?

(i hope you can understand my english)


Make sure you put the needle back together correctly. I had a similar problem with my bike. Found out I put a washer in the wrong place and the needle was higher ,than it was supposed to be. It was getting to much fuel ,backfire and run rough at idle, but ok when I got on it. I tore the whole motor down trying to find the problem. Found out what the "simple" problem was after a lot of time, and head banging.

I should add one thing to my previous post, My 426 also started backfiring, hardstarting, fouling plugs, etc. after it was about 3 years old and I ended up replacing the stock CDI with the Vortex X-10 version and all my problems went away. I too went through every possible problem that could have caused the issue with no luck. Oh yeah one other thing to make "SURE" that you do is clean the ground connection for the coil, it may be rusty and corroded under the bolt. Hope this helps -- WR Dave. :banghead:

thanks guys for all the info will give a try thanks again.

For what it's worth, I had similar problems. Backfiring (when trying to accelerate past midrange) but fouling plugs.

Traced it to a bad connection between the spark plug cap and the lead going to the plug cap. This was after six months of stuffing around with jetting!!! Hope it helps.

thanks every bit helps

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