RMZ 250 Sputtering Problem

I have a RMZ 250 and at really low RPM's it runs fine and if your wide open on the throttle it runs fine. Now my question is the MIDDLE range just sputters like a 2-stroke. I've tried a few different jets in it. But i was wondering what the different jets do (the Main and the pilot/slow jet) if each one is for a different range or something. Any help would be appreciated.

Thanks  

leak jet as well..
DavidS  
i take a lot of pride in what i am

___________________
ride: 02yz426
local: north east Oregon

terrain type woods with lots of roots & rocks

Should i start with the stock jets?? And What is a leak jet i've never heard of it. Does each jet control a certain range of Power or something???

Thanks for the help  

Bump  
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"Every time I start thinking the world is all bad, then I start seeing some people out there having fun on motorcycles. It makes me take another look." -Steve McQueen

yur bike should=n't sputter in the middle. Sounds almost like its not getting enough fuel or air. But what do you mean by sputter? Like its almost going to stall? I'm not really sure what to say without more info  
1978 Yamaha 175 MX
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Its like when a 2-stroke aint in the power band. they dont have a smooth sound. that is the best way i can think to explain it. I'm pretty sure it aint gettin enough gas cuz i checked the spark plug and it was white. Thats why i think its the jets. i just bought a jet kit and i'm going to try em out.  

Well i've been trying some different jets and moving the needle clip and i've bout got it fixed. Heres a sight that i found that gives a lot of information on Jetting.

http://www.4strokes.com/tech/howtojet.asp  

I have a 2007 RMZ 250 and i have the same problem... Ive kept re-jetting for the past couple months and have been working on it constintly but cannot seem to stop the sputtering problem in mid range. i have bumped the main jet all the way up to a 182 from the stock 162 and have been moving the needle but cannot seem to make the sputter any better. i have worked with the air screw all the time but nothing seems to work. let me know if you have any luck with your jetting quest and if so please let me know what you did so i could try it. Thanks.  

Boyseen makes a new accelerator pump, and a power wing. But like the guys said the pilot(small jet) needs a look as well. It helps meter gas + air up to about 1/2 throttle.  
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I found another website on jetting it has a chart that shows you what jets/needle controls what throttle range and some other info.  wish i would've found that website before i started this quest lol

http://www.speedandsport.com/jet.html