HELP Bike Stutters On Mid-full Throttle 45/50mph

[MattisJesus]

Hey all, I have a yamaha Wr125 x. It has 21k miles clocked and I have owned the bike for about 4 months as I got it off my brother who has kept it in a moderate (if i'm being nice) condition. The only changes I have made to the bike from standard are putting K&N Air fliters in.

Recently the bike seems to stutter when I either accelerate hard, or use full throttle no matter what gear I am in. I just recently changed the spark plug and that has had no effect. It seems to be ok if I ride the bike cold for about 10 mins maybe stuttering once or twice, but when it warms up it tends to stutter often. It also stutters around hitting 45/50mph.

Any ideas what this could be/what other information do you need?

[_mjs_]

Have you checked the powerband?
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Previous Bikes: 2006 Honda XR125L > 2003 Yamaha DT125R > 1996 Honda CB500
Current bikes: 1997 Kawasaki ZZR 600 E5 > 2006 Honda Varadero XL125V

[sidewinder]

I'd try putting the old filter back on to see if that helps.sounds also maybe your coils on the way out aye even open the fuel cap breather could be blocked..

[Rogerborg]

Definitely put the standard filter back in.

What colour was the old plug when you took it out? Have you done a "plug chop" on it now? That means ride it at full throttle for as long as you can (while it's acting up), then clutch in and hit the kill switch, come to a stop, and then (carefully) take the plug out to have a look at it.

Compare with this:

https://members.modernvespa.net/fabio_dougie/uploads/spark_plug_colour_810.jpg

If it still does it with the standard filter, then I'd suspect that the lambda sensor is faulty or disconnected and it's running rich. But diagnose, don't guess.
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Biking is 1/20th as dangerous as horse riding.
GONE: HN125-8, LF-250B, GPz 305, GPZ 500S, Burgman 400 // RIDING: F650GS (800 twin), Royal Enfield Bullet Electra 500 AVL, Ninja 250R because racebike