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04 KX250 2Stroke Revs to the moon when kickstarted...

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Hello all, I have a 04 kx250 2stroke.  Just rebuilt the bottom end got it all back together.  Went to start it up took 2 kicks and it fired right up.  Was idling somewhat good for about 4 seconds then revved way up in the rpm range and got there quickly.  I shut it down with the kill switch.  Looked it over everything seemed to be in place.  Tried again and this time it fired up and went straight to the top of the rpms. So, then I pulled the carb cleaned it through and through.  Made sure floats were adjusted and carb clean blew through all the right holes.  Everything is moving nice and smoothly and completely clean.  I then went to start and this time the bike did not want to start.  Took a whole lot of kicks and a ton of energy finally got it to fire and to my utmost frustration it again went revving to the moon and then my kill switch had a momentary lapse in functioning as it did not kill the motor right away.  But, finally did it's job and got the motor to shut down. 

 

At this point I just don't know where to look.  Could this be a different issue like power valves in wrong position? Air leak into cylinder somewhere (thinking that would bog it down not rev up), Bad tps sensor? Throttle cable? Even though moves freely. Any help would be greatly appreciated!! Thank you.

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I did not pressure check it when done.  It definetly has compression but unsure right now.  You think it would rev up if it's to low/high in compression?

 

And do you mean behind the stator? Blackwoodz

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You "just rebuilt the bottom end" by this I assume you installed new bearings and seals. A seal could for sure be bad and that would/could cause the problem. compression wouldn't cause the problem you describe unless it was way wore out and you put in a new piston and freshened it up, in which case I would say that all you need to do is turn the idle down. when foirst started there was, I hope, alot of oil and once it burned out it started reving. by the way if they get to revving uncontrollably the kill switch may not kill it, you have to put it into gear and stall it. there really isn't much information to try to diagnose a problem, for example did you do the rebuild on the bottom end, have you done it before, did you use new bearings and seals, did you do the top end as well what do you have for jetting is the idle screw way in  should try to include a bit more info to get meaningful answer

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If it only revs to the moon when you have the choke knob pulled for cold starting, and runs/idles fine with the choke off, your air leak might be in the choke plunger assembly. I had that experience with my '00 KX125 and it was a simple fix.

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I did not pressure check it when done. It definetly has compression but unsure right now. You think it would rev up if it's to low/high in compression?

And do you mean behind the stator? Blackwoodz

Time to pressure test it. You should always do this before putting it in the bike.

This will tell you if you have any air leaks

Pressure testing has nothing to do with compression.

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I had it once while I was riding a Enduroday. The bike also revved to the moon :)

I blew out the gasket which sits between the reedblock and the engine case, due to the bolts came lose which I didn't torqued down.

So I think you should look for an air leak...

Edited by Daniello

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Just a thought here. Since it's been apart one thing I would check is the routing of your throttle cable. Make sure it's not binding under the tank or by the top of the forks. See if the idle changes with the handlebars turned in either direction.

Edited by jcm2112
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