04 WR450 stumbles bad at mid rpm or when I rip the throttle.

I just picked up a WR450 2004. First thing I noticed is it would stumble bad when you accelerate as soon as it gets midway up the RPM range. I got for a good price so I didn't care.


I pulled apart the carb and cleaned all the jets and everything else with carb cleaner only to find the rubber on the float valve was melting. I put it back together and the bike started up first kick every time and idled alright, but the stumbling still occurred ad mid rpms. It was also leaking gas. I think the melted float valve caused this.


Brought the carb to my local shop and told them to replace everything rubber.

150$ later, I get my carb back and install it. Starts up great, idles great but still sputters bad if I rip the throttle or if once it gets midway up its rpm range.


I noticed all the jets were richer(bigger) than stock and the needle clip was in like the 4th or 4th position (I forgot now.) The accelerator pump mod (where you tie a wire around the spring and the little arm) was done. The exhaust looks stock.


Is it possible the bigger jets and the clip being so rich is causing all of this? It has an adjustable fuel screw and I played with it a bit but it makes no difference in the mid RPM/full throttle stumble.


I am spending too much  time and money on this haha. Any ideas?



Thanks a lot.

Brian Dady

Have a look at your plug. If its really sooty you need to lean it out.

Jetting might be totally wrong, prob why PO sold it !

Not too difficult, just only change 1 thing at a time.

This isn't a bog; its more like the carb is giving me the middle finger and wont go past like 4 or 5k rpms.


I just pulled apart my carb (for the 3rd time in a week sigh) and here are the jet sizes:


Jet Needle: 4th notch (correct according to shop manual) Jet needle is probably stock.

Fuel screw: Adjustable fuel screw - I installed an o-ring and spring after I lost them during disassembly. Washer still there. No matter what adjustment, stumble does not go away.

Needle Jet with main jet screwed into it as well as another jet thing screwed into that: Needle jet has no number, main jet number appears to be scraped off so no number, and little jet screw thingy screwed into that has no number....

Starter jet: No number

Pilot jet: #50

Leak jet: 60


Accelerator pump seems to be fine. Squirts nice and the diaphragm is good I think.



Did I miss anything? All the numbers are scraped off. grr



Im going to buy all new jets tomorrow.


Well you need to decide if it is a rich or lean stumble before buying jets.

Could be a bad coil so jetting might not help.

New plug, Ride it hard and then check your plug.

I had a 400f used to do that when needle was too rich.

Does it run good at high rpm? If it does then just play with needle height. I would raise it to start and see how it runs

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