03 WR450 engine miss.

I have an 03 WR450 and love it except for one thing, I ride on the trails and on the street, when cruising at a steady pace the engine seems to studder, I have been told that grey wire or throttle position sensor mods will solve this problem, maybe a toggle so I can turn either on or off?? I would like to be able to cruise the roads and dirt roads without the miss. I do have the YZF exhaust cam installed and I would never go back to the stocker, but it does seem to magnify the problem a bit. Any help would be appreciated. Thanx. Dirtmizer. :)img_0303

Sounds like jetting to me. My bike is also street legal and does the same thing depending on the weather. :)

I just bought a 01 wr426 that is streat legal, and its doing the exact same thing. I'm gonna try and drop the neddle on space and see if that cleans it up. will let you know how it goes

Disconnect your TPS for better steady speed throttle control. Draw back is you want it connected for off roading! :)


I've got both the gray wire disconnected and the TPS unhooked and love the way it runs. Why do you say NOT for trail riding on the TPS?


The TPS helps in low speed technical riding only. It is a hinderance for steady throttle/ speed street riding. If you dont ride nasty technical terrain you may never notice the advantage. :)

The TPS helps in low speed technical riding only. It is a hinderance for steady throttle/ speed street riding. If you dont ride nasty technical terrain you may never notice the advantage. :)

Thanks for the tip!! I will have to try that out.......!! :)

I have heard about the TPS and I would like to set up some kind of toggle so I could switch back and forth on the fly, unless I can just leave it off. I do ride tight technical trails fairly often so maybe I need it for that. I will look into both the grey wire mod and TPS. Thankyou. :)

OK, my 2003 WR450F runs awful at constant throttle. There is hardly any midrange power. How do I mod the grey wire and TPS?

I had the same problem, but I solved it by closing off the ACV.

There is a spring that richens and leans the mixture depending on how you are working the throttle.

For example, when you close the throttle from high rpms it richens the mixture to avoid backfires.

The problem is when the throttle is held steady the spring leans and richens the mixture chaotically, causing the engine to surge. It's not really a miss, its a surge-on, surge-off situation.

Indy knows the drill on the 03, I'd follow his advice. I think the real culprit is the little air valve on the side of the WR carb (is that the ACV valve??), it's also the thing that makes the WR rev down more slowly when you cut the throttle than the YZ does. I think it's on the WR carb to help it meet polution standards. You'll notice that the YZ450 and the CRF450 don't have it. I ruined my stock carb and installed a CRF450 carb and it runs perfectly smooth at steady speeds and revs and backs off instantly. I hope I'm not full of BS on this but I think thats' one of your problems.. I'm not advocating you get another carb, doing the mods and correct jetting will make it run fine. :) Big Jim

Sounds like the grey wire.

I would try cutting it first and move in small steps to see if you can fix the problem before trying more involved measures to solve the problem.

In my experience (00 WR and 04 WR) they both had a stumble on constant throttle openings. I didnt notice if much off road, mainly cruising on dirt roads when the throttle was constant, it would surge and stumble, highly annoying when traveling any distance

Mine was doing the same thing, today i dropped the needle on clip, went from the 4th from the top to the 3rd from the top, world of difference, doesn't sputter and pop at mid throttle anymore. It was easy to do, I didn't pull the carb off, I just turned the handel bars to the right to give the throttle cables some slack, loosened the carb clamps, and twisted the carb so I could get the top cap off, once the cap is off, there is a small allen nut that holds the needle in place, remove that nut and with a pair of needle nose pliers just remove the needle. take care when removing the clip as its easy to lose

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