i just got a 92 kx125. the thing idles fine and revs fine until a certain rpm. at first i thought it might just be a carb problem so i took it apart, cleaned out and went with a little bigger main jet (it had a 158 now has a 165) but made no difference. this isnt just a bog really its like the thing just quits pulling altogether. i also checked the powervalve and it seems to be functioning(its not stuck anyway) so now i am really leaning toward the stator or cdi. the reason i wondered about the cdi is because its like its hitting the rev limiter or something. i am by no means a 2 stroke expert but i can do about anything mechanical since im a mechanic. but this thing is whooping me. any help would be appreciated and if someone thinks it could be the powervalve please explain what i need to look for. thank you for any help you can offer  

My first thought was eather main jet, or power valve.
The power valve would be a more MID to wot issue.
I'm wondering what your compression is?
When was the last top end?  

i dont know what comp is or when the last top end was, i just got the bike. this thing quits pulling as quickly as you could flip a switch. do you think that could be compression? i will check it and see. any other ideas?  

Comp is compression. Sounds like you need to do a top end! No big deal. The reason your not getting top end and the hit is your bike is probley due foe a top end thats all.  

What - Woods ideas were'nt enough for ya?
Well - my first suspicion would be a worn topend. If you don't know when it was done last, it's time to take it apart.
158 to 165 is a pretty big jump on the main, I'd pull it back to a 160 or 162.  
I doubt it's the CDI... when they go, the bike just won't run.  

I'd be willing to bet that your comp is WAY low.  

I'll take some of that action! I'll bet it's WAY, WAY low!  

Din't know you were up Skip, So why am (I) fielding these questions?
Me,Tigger, and my smilies just stopped in to run rampped alittle! LOL  

Hey - I posted to the Night Shift Roll Call, Mr!
What am I doing here?  What are YOU doing here!  

yah i know what comp is i just meant i didnt know what the compression of the bike is. i will check it tomorrow and see what i get. you guys are prolly right its proly a bit low but with the way it was acting i didnt think that was it. anyway thanks for the help. what kinda #'s should i look for on the comp test  

165+ psi at sea level.  Personally I prefer to measure the ring end-gap when determining the condition of the topend.  

Before you tear into the top-end, I'd check the carb slide to make sure the spring or the cable isn't getting hung up.  There is a plastic clip below the slide spring that tends to get sideways...or could be installed incorrectly.  Check that the clip is in the needle as well.  Then, check the float level and vent hoses.  I'm trying to think of other things that you can do without major disassembly...plus, I think if the compression was bad, it would be hard to start and run better at high rpm.  

well im gonna check compression tonight. but one thing i can say is it is very easy to start.  it sat in the garage for 2 days without being ran and started on the first kick. the bog doesnt seem to be dependent on throttle position, just rpm. i will check the things you mentioned before tearing too far into it. i did take the carb slide out and adjust the needcle clip to see if it made a difference and got no better or worse. also if i do have to rebuild top end can someone tell me what size is stock piston (mm) so i know what to order. also would i have to do anything to the powervalve if i bored it .20 over stock?  

for your piston you need to get the old one out and take the numbers off of it so your can order the next size needed...figure out your top end and rpm problem before you start messing with other stuff on the bike...just what I'd do.  

ok so the verdict is 100lbs of compression. looks like you guys were right on it. dont know if there is a specific way to do it but i had to kick it about 6-7 times with throttle wide open to hit 100. i took it apart and looks like the piston is a stock 54mm. where is a good place to get a piston kit for reasonable price. ebay didnt have much for my year. also since the cylinder doesnt have any bad scarring should i just go .20 over or more? thanks again for your help guys. anything else i need to know will b helpful