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This is going to be very long winded, but I know there are people who know a lot more about 2 strokes than I do that I need help from, so I want to provide as much detail as possible. I bought a 2003 KX 250 about six months ago. When I first got it, I replaced the air filter, cleaned the carburetor, and put in a new spark plug. Up until about a month ago, that was all I had done to the bike besides minor stuff that doesn't relate to the actual performance of the bike. It rode hard and fast since I had bought it without a hiccup (besides some fouled plugs), but on a ride about a month ago, I had been riding it decently hard for about an hour and a half when I tried to really open up coming up a slight hill and it sounded like the bike hit a rev limiter. It still had all of its low end power and otherwise ran perfectly fine, but it was like it could not get into the powerband. After keeping off the pipe and letting it coast as much as I could on the way back home, it seemed to get a bit better, where it would just get there, but any farther and it would sputter like crazy and bog. Just two weeks ago, I came to the conclusion (to the best of my knowledge at least) that the power valve was sticking shut, and since I was going to take the cylinder off to clean the power valve anyways, I decided to rebuild the top end. After cleaning the power valve, replacing the piston, and repacking the silencer, I broke it in nice and good and let it rip. It sounded crisp and had all of its power everywhere it was supposed to have it. I felt like I was riding a new bike, so I enjoyed that for about half an hour. Next day, I hop on it and warm it up, take off and it bogs when I get to the powerband. Every time I've gotten on it since, it's a different story. One minute, it's bogging like heck, I sit for a few seconds to calm my frustration, take off and it rips with a sound like that of angels singing. It always runs exactly the same at low rpms, but very different at high rpms. I hate to admit it, but I cleaned the power valve without completely disassembling it, so I think that may be my problem. I'd just hate to tear the bike apart again, thoroughly clean the power valve assembly, put it back together and then my problem still not be fixed. Not sure if it's important, but it does foul plugs quite a bit. It has since I got it though and it ran good before this problem started. Any help is appreciated. Thanks for reading.

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clean the carb like you have never cleaned it before! mess with the air/fuel screw and if that doesn't work try rejetting it. also If you have a spark arrestor, I would check that to see if its got a bunch of build up on it, that was my issue when I had a very similar problem.

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Cannonball.z ,

your jetting is way to rich if it is fouling a plug at all EVER.

you might have a cdi or stator  problem as well 

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Clean out the filter on your gas petcock, they get full of gunk.

Also make sure your power valve is operating correctly. I noticed on my right exhaust sub valve was hanging a up a bit when I rebuilt motor.

Clean the carb and jets very well.

What are your carb settings?

Also just double check your timing mark on the stator. Center mark is a safe bet.

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Haven't had a lot of time to mess with the bike recently. Carb is clean, spark arrester is clean, filter on the gas petcock is clean, power valve has been deeply cleaned and is operating correctly, I'm not sure on my carb settings right now but will check when I'm not so busy. I've also never dealt with a stator on any vehicle so I'm not really sure how to go about checking that out. I have a video of it running but I can't figure out how to post it (if I can). Thanks. 

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Here's a video of my friend riding it. It sounds alright through first and second, but into third and through fourth, it does its thing. 

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I had a problem similar to this when the TPS(throttle position sensor) on the top of the carburetor wasn't operating properly.  I had taken the black TPS connector off the carb, there is a little spring loaded wheel inside the connector and it unwound when I pulled it off and I didn't notice.  When I put connector back on I lined up the slot in the wheel and the tab for the arm of the TPS but had no tension on it so when you were riding it would bounce up and down confusing the cdi.  It wasnt super noticeable in the lower rpms but up high it ran like crap most of the time, in a few instances it seemed ok.  You can check it easily by taking off the top of the carb, pull the slide out and make sure the arm snaps back when you pull it up.

 

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No, stupidly I don't know my jetting. This is my first two stroke and I didn't realize how important jetting was until after the last time I cleaned the carb, so I don't have anything written down. 

 

I checked the TPS and the arm had no tension. I pulled the TPS connecter off and it just spins in circles with no tension both ways with a scratchy sound inside of it. I'm assuming that's not good?

 

(edit) If it means anything, the day this all happened, I had been riding around a lot of snow and puddles, and I feel like I've heard of water damaging the TPS. Is that true?

Edited by canonball.z

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7 minutes ago, canonball.z said:

No, stupidly I don't know my jetting. This is my first two stroke and I didn't realize how important jetting was until after the last time I cleaned the carb, so I don't have anything written down. 

 

I checked the TPS and the arm had no tension. I pulled the TPS connecter off and it just spins in circles with no tension both ways with a scratchy sound inside of it. I'm assuming that's not good?

I will almost guarantee that is your problem.  The spring is probably broken in there, you'll need a new/used replacement.

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Okay thanks. I just added a bit to my previous post if you know anything about that. I'll look online for a replacement. 

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