Wr426f misfire at 1/2 to 3/4 throttle on cruise

Hi everyone this is my first post,I have a wr426f 03 & live in the uk. It has a hindle full exhaust system & twin air filter,All runs fine apart at 1/2 to 3/4 throttle on cruise it will miss. I can accelerate through or past it,or back of slightly all is fine. It has standard jetting & plug color is as it should I've cleaned carb to.

Any help would be much appreciated

ThanKs Daz

I'm assuming that what your talking about is a minor miss, right? As in, a slight surging sensation? If so, this is normal. It's the function of the TPS system. These dirtbikes don't like steady throttle settings.


TPS like Maniac said. Check out the stickied posts at the top of this forum for info on how to adjust it.

Hi again after stripping & cleaning the carb still the same,definite feels like a misfire but One beat every few seconds. It's on standard jetting & needle,could this be a jetting issue? I know the plug is ok & sure is fuel related,I'm thinking of a jd jetting kit.

Thanks again.

I consider my bike to be as perfectly jetted as can be and I (and many, many others) have the surging you are describing. I have never bothered to adjust the TPS (Throttle Position Sensor) since I rarely ride on anything but singletrack. Start with the TPS as it is the easiest and quickest thing to do...just unplug, ride, see if there is a difference. Then follow the sticked posts to adjust if necessary. After you have done this, and it is still irritating you, then look into jetting which really won't fix it anyway.

Hi again,I've taped a quater of the air box intake up & rode it at operating temp & the conditions to get the fault & it's gone. In fact I'm sure it rode with slight more power through all throttle positions. I did disconnect the tps,that seemed slightly better but not as good as the air intake taped up. So I think could be a jetting problem,or I could try lifting the needle a notch in the carb.

Any help is much appreciated thanks again Daz.

Sounds like you have/had multiple things going on at once. If you have restricted the airflow, the jetting is way too lean as you previously had too much air.

To have the full power potential of the bike, open the airbox back up, and then re-jet properly. Make sure you have the proper pilot air, and main air jets for your bike (YZ is different than WR because of the ACV). If I were you, I would disable the ACV circuit and use the YZ pilot air jet. I did it and it makes tuning a lot easier.

Then work on the fuel jets starting with the pilot jet and main.

The needle and needle position will have to be adjusted somewhat based on your feedback. I tried both JD needles (from my kit purchased in 2002) at different clip positions until I found a combination I liked. I think I wound up using the blue needle (cold weather) year round, even in 100 F weather as it seemed to run the best.

Follow the stickied guides to help you along the way. You will have more power than you had with the airbox taped off without being lean like you are now. With every mod you do, you will probably have to tinker with the jetting some as the relative ratio of air to fuel will change a bit.

Finally, in my experience the very best free-mod you can do is the O-ring mod if you haven't done it already.

Edited by Pooley

Thanks for your help,jd jet kit on its way & fuel screw.



Minor hijack here

Just fixed my ap circuit , can now crack open throttle and actually have power.

and now have VERY ROUGH running at 1/3 - 2/3 throttle.

Full throttle is awesome. Idle is and very low is ok.

I'm thinking tps might be playing up so I'll try unplugging it next ride.

Anything else make it run like crap at mid throttle?

needle position should be your midrange, could have come lose, could be needing adjusting? could have an air leak that is bad, but not so bad that the main jet can't spew enough fuel to hide it.

Last week my dealer did the AIS removal, installed the jets/clip included in the Yamaha kit, removed the baffle, air filter snorkel, and grey wire on my 2009 WR450. Took it for a 45 mile ride yesterday, 800' elevation, crazy hot day for us in Wisconsin, 98 degrees. Idle-good. 2/3 throttle or more, great, any thing else, stumbles-misses-sputters. Never had any issues before the mods, would the mods have triggered TPS issues? And idiot question follows, where do you unplug the TPS? Thanks much,


The TPS wiring comes off from the left side of the carb and goes up to a connector about 4 inches above the carb. You can dis-connect right there. I would think that your hot weather was the cause of your bike running poorly, not the dealer's fault. That's my guess anyhow.


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