HELP! Miss Firing KX250

[KawiKid]

Hey all, just got my KX250 back from the shop yesterday and I took it out for a short ride to see how things were- new power pipe and muffler all jetted to suite.Everything was great but when I went out on a long ride this morning I got about 15ks away from home and every time I hit power band my bike would splutter out so I had 2 ride home just putting all the way the problem started about an hour into riding.
I have absolutely no idea what could be wrong with her I checked the plug its fine, I checked my reeds to make sure that they weren't broken causing my engine to flood Im starting to think it could b my fuel mix or power valves??
Any ideas would be great
Cheers
Kiddo

[Hillclimb#42]

 Did it idle just as before? Cut out with throttle tapped? Could easily be crap in the main jet. Too slow of fuel flow somehow. Good to have checked reeds. How old is the fuel? Probably that main jet. You can remove it from bottom of fuel bowl. Be ready for gas to come out. Pull out the jet and look through it. Should be able to see light. I always blow them out onto paper, so I see what was in there. Then you may see what you need to do to prevent.

[KXcam22]

Kiddo,
 Since it was running good for the first hour it sounds like you may have picked up some crap in the mainjet, or it has plain fallen out (I had that happen to me once - strange as it sounds).  The power valve issues will make it lose power at the top but it will still run ok.  I like to run those mini inline fuel filters, always have and have one on every bike even my CBR900.  Avoids lots of potential issues for a $2 filter.  Keep us posted on what you find. Cam.

[KawiKid]

It is idling fine it only cuts out in power band. The fuel is about 3weeks old probably less then that and I always give it a good shake up before I top up my tank. Could it be the power valve because it only seems to cut out in power band?? My jets is where I'm planning on looking next because I have had the Bike re-jetted to suit V-force reeds and PC pipe. Will check the Jets after Xmas Lunch or at a spare moment. Cheers guys

Merry Xmas
Kiddo

[KawiKid]

Found the problem guys. Supposedly brand new spark plug was just a cleaned second hand one, me and my cousin discovered this on closer inspection. Because we tried everything you suggested and eventually my cousin rode the 250 and came back and said definitely spark plug I don't know how he knew but we changed it and she is running sweet now. Im just lucky I guess

Cheers for the help guys
Kiddo