99 400 sputters on mid-top end?? help

I am trying to fix my friends 99 wr400. It starts and idles great, takes off nice and when you get over half throttle it sputters like the ignition is cutting out. My friend tried a new cdi box and that was not the problem. I tried it with the neutral safty unpluged and the TPS unpluged and it is still the same. If you got any ideas let me know, thanks

Is it jetted correctly? Sounds like the main jet; pull the plug and check if it is sooty, indicating a rich mixture. If it is, go to a leaner main jet.


ok this may sound funny but one of hte wr mods was the grey wire and the trottle stop check to see if the throttle stop has been ground down the 8 mm to remove the throttle stop if so check to see if the grey wire has been cut according to the mods if you remove the throttle stop you have to cut the grey wire so the voltages for the powercurves can be computed correctly! best thing to do is look at the mod this link is for the 250 but is the relatively the same for the 426 the 400 and the 450 look at throttle stop and the grey wire!


This bike was running fine a few weeks ago, it just started running like crap. It seems to me like an ignition problem. The funny thing is it starts and idles perfectly, if you ride it very easy you wont notice it, but as soon as you start riding it normaly and over half throttle it misses and bucks.

I shut the gas off and rode it until it ran out of fuel and it missed until the very end. I would think it would have breifly run good if it was jetted to rich. so i am convinced it is something to do with ignition.. any ideas?

Have you tried a new coil? Sometimes they'll crap out and cause symptoms like this.

Ok so I was wrong! It was somthing with the carb, When I ride it with the air box cover off it runs perfect! So it was starving for air with the cover on the air box. What would cause it to randomly start starving for air? some how it must be getting too much fuel ? keep in mind it was running good a few weeks ago..


how dirty is the air filter? my ktm did the same thing and I cleaned it and fixed the problem.

the air cleaner is perfectly clean.. on the float should i just check the level? if its running rich would that indacate the float level it too high?

sticky or worn Needle valve......float......worn needle......are things I'd check.

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