poor performance/back firing

Help ... depression is starting to set-in on my 01 WR 426. Here's my story ... I bought it new in Oct 01 and rode it stock for a month (warranty period). Then I did all of the standard mods, such as; throttle stop removal, grey wire cut, exhaust tip removal, and re-jetting. It ran great for six weeks or so until one cold December day at the local track, the bike would not start. So back home I went. Checking the plug I found no spark. A new plug resulted in a second kick start but the bike now back fires really bad and performance has dropped off. It seems to miss at all rpms. At idle it's rough and at mid throttle the miss is real noticable. But the back firing is excessive ... I'm afraid to ride it in fear of doing major damage. Here's my current set-up.

168 main, 48 pilot, 100 pilot air, 1 1/2 turns out. It's been 40-45 F degress and high (70-90%) humidity. My location is 500 above sea level.

I've tried different jetting (including stock) and have played with the air/fuel setting/jets, new plugs, different plug gaps, new gas all to no avail. The plugs are not rich or lean. I've talked to a lot of peolple and have read all the posts on back firing, but nothing seems to help. Has anyone experienced a sudden change in performance similar to this? Has anyone experienced electrical issues? Any advice would be appreciated.

Greg (turp426)

01 WR 426

Greg, I'm no jetting god, but I have a similar issue sometimes...Might be a CDI issue or who knows? My '01 WR runs great except - sometimes when I first start the damned thing it does what you describe. Usually if I shut it down and restart, it'll run fine (I mean great). I believe it has something to do with the enricher. It's a circuit in the carb that's used when I pull the choke on the kehin carb. I first learned about this type of enrichening on my harley. It's an air ( or fuel) probably an air circuit on these carbs. Anyway, I always pray (yes, i do that) it'll fix it self and it has so far.

If you bought it new, then I assume the electronics are fine, but I recommend you pull the carb and go through it. I mean soak it in cleaner and blow out EVERY passage with high pressure. Seems like our bikes have real carb issues until you get it dialed in. Never hurts to know you bike's innerds.

When you solve your problem, give my a holler at my e-address as it might help me too.


BTW, took a vacation in the SeaTac area MANY years ago and caught some drag races too. They were having a drought, so it was pretty nice up that way (remember we get <9 in. rain/yr. here in ABQ.)


Have you checked the sparc plug cable? I had problems with plug fouling and poor ignition, put after cutting a piece of the plug cable and screwing it into the plug hat again seemed to fix that. Just a thought, something really easy to check...

Check the inlet and the exhaust for any leaks. Check if the fuel tank ventilation isn 't clugged.

[ January 08, 2002: Message edited by: Ynahg ]

I would be interested in knowing what the problem is as well. I have an 01' that was bought in Sept 01 and it is missing when I first start it as well, after it warms to operating temp or I restart it, it seems fine.



Thanks for the suggestions. But my bike never makes the turn for the better, warm or cold. Even after more than 10 or 15 minutes, it still runs rough and back fires. I guess what really has me stumped is that it was an over night change without any modifications to the bike, from running perfect to absolute crap. I've pulled the carb and have cleaned it out but it wouldn't hurt to do it again before it goes to the shop (which I don't want to do). But if it is electrical (CDI) I'm hoping they can find it qiuckly.

Thanks agaun for the the input ... anyone else have anything like this?



have you cut the right grey wire?

Before you take it back to the shop it is fully worth putting the jetting back to stock.

What is your 'newly' installed sparkplug telling you? is it black, white, tanned?

You make no mention of the carby needle, is that still at stock setting... Personally I think your running rich.

before you take it back to the shop you can do yourself a favor by starting from the begining again -

clean and reoil the air filter

check the sparky, replace(or reseat in your case if it's new)

return the carby to stock jetting(including air)

+ the needle, and eyeballing it's in good condition

also eyeball your float needle still exists

Kick it over and see if it works well.

I take it you haven't gone to YZ timing or added flywheel weights yet.

Greg, Check this. I went riding one day all of sudden I had all sorts of problems with backfiring. Changed my air Filter a couple of times at least thinking I had the filter to Dry or Wet. Worked my Pilot screw in and out it seems like a hundred times only to leave it where it had been set by the factory. Dumped my gas out and went out and bought some fresh Gas from another source filled it up "Still Back Firing" Dumped the gas again and cheked the pet-cock filter by removing it. Changed my Plug and tried to Gap it more than most guys would do in a year. Called the Yammie Shop. Guess what they told me bring it in. Called a Friend he said let me call a buddy. He called me back in a flash and said.

"You didn't happen to adjust your IDLE did you"? Well it never occured to me through all of this but Yes I Did. Bike seemed to be running a little on the Low Idle side one cool morning so I turned it up a bit. That is where it all began and that is where it all ended.

Hope this helps.

The only problem this bike has ever had is me.


WR 426 2001

TTRL 125

PW 50

Greg even if you did not turn your Idle up. Turn it down and try it. Can't hurt. Let us know what you find.


Ike of Colo

Sorta sounds like a valve is sticking open,or an electrical problem (TPS?). The CDI's have not been known to bad on WR's.

Does it just pop/gurgle or really backfire like a gunshot? If it pops/gurgles-probably jetting or obstruction in carb/instake/exhaust. If it sounds like a gun shot, could be a stuck valve. You're bike is so new, though kinda doubt it's a stuck valve.

Also what needle are you running? If you're on the stock needle, I may suggest dropping the PJ to 45. Alternatively, a DVR needle might be a good choice (for WR timed) or EKP/EKN for YZ timed.

Good luck, and please let know us know what is causing the problem once you find it..

After being sick for a while, I have finally had some time to find out the problem. And it took a little luck to find it. I was checking for loose bolts, nuts, etc and decided to check the two ports on the left side case cover (CDI magneto cover) and sure enough they could use a little tightening. But first I wanted to see inside. As I unscrewed the first one I was greated with ENGINE OIL, lots of it. I'm sure this is a bad thing as all of the electronics are submurged in oil. The manual shows the timing chain directly behind this, and it seeems that this should be wet. The manula implies that it should be quite dry behind the cover, like all of my other 2-strokes. Looks like it could be a seal, but there are several that could be the cause.

I will be calling the shop I bought it from (Florence Yamaha) and see what they say. Has anyone else heard of this or experienced this?

If I had to guess, I would say that its pretty cold and maybe even wet where you are located. If this is the case. Lean out your fuel screw, turn it in 1/2 turn at a time...

Have a great day!

Mr T

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