Kawasaki Dirt Bikes Forum

He is losing power to heat some how.  It runs fine warm but as soon as it gets hot he loses power.  It does not sound like the bike is overheating or he would be telling us about glowing pipes and the buning feeling in his legs.  I don't think it's jetting as it runs fine at first.  I don't think its a clogged filter (air, oil, or fuel).  I think the bike is droping serious compression when the bike heats up.  A compression test will tell us this.  I am open to any other ideas you may have.  

Bill H. 2000 CR250R

Is what my bikes doing the symtoms of valves being to tight though? I guess theres just some many possibilities.  
Pain is temporary, Glorys forever.

I don't think too tight.  I'm thinking worn out.  Worn valves or valve seats or both.  Re seating is commonly done on these bikes and usually you get a better job than the factory initially gave you.  This is also a good chance to have everything polished up.  It will be expesive,  but if you get new valves, new seats, new cam chain, piston and rings your bike will run as new.  You may even be wanting new cams to improve performance.  The stock KX250f engine was a favorit amongst vet riders for its low end power and willingness to pull a taller gear than the other bikes.  The trade off was less top end, but most riders don't get into the top end often.  The chassis was not the best but still plenty good for most of us mortals.  I would say it is probably worth fixing up this bike.  It will give quite a few years of good riding as long as you stay on top of the required maintenance.  You just bought an older fourstroke motocrosser that we try to shy people away from buying.  This is the downside to fourstroke MXers.  Otherwise I would welcome them with open arms.  

Bill H. 2000 CR250R

So basically i should trade in the 04......for an 09.....haha i wish...  
Pain is temporary, Glorys forever.

I'm running out of ideas too, Bill...

To answer the question - yes, tight valves can cause stalling..

Worn out seats will leak compression - that's for sure!  
Tight valves do the same thing.  As the engine heats, and the metal expands, the clearance between the lifter and the cam decreases. That's why we leave a few thousandths of an inch clearance.  If you could measure your clearance while the engine was warmed up and running, you would find that the clearance was down to a thousandths of an inch or so.  Now if the valves start at zero, then the're being held around .006-.007 open when the engine is running! Bam - Compresssion loss. You don't experience it when the engine is cold, because the metal hasn't expanded enough to use up the existing clearance.  However, when the engine warms up fully - the metal expands, and the compression starts bleeding off through the now-permanently-unsealed valves.  
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Here we go again!

I know I may not be the most experienced around but I gotta ask a few questions...

-When you say it dies after 20min or so...do you mean 15-20min? or is it realistically more like 5min? ( I ask this cause I would think after 15-20 or serious riding, this bike would be more than "warmed up")

- If you are running at higher rpm does it die faster than if you are running at low rpm?

-When it dies are you at a mid-high RPM or are you just idleing or does it vary when it dies?

- When you say it "dies" are we talking about it is prring at mid rpm and then straight cut out, or is it more like a 'mid rpm - low rpm - dead' type ? Even if you hit the kill switch at higher rpm it doesn't die instantaneously, it still takes a second to cut out completely...


-  

Stator was bad...  
Pain is temporary, Glorys forever.

Interesting...  Did they give you an explanation how a coil would only act up after heating?  

Bill H. 2000 CR250R

no $hit...wouldn't have guessed that...well at least its fixed.. right?  

I spoke with my friend last night.  He is a Dyno technician who has worked for some pretty big companies, Roush being one of them.  I asked him about your engine and he guessed stator before I could mention that was the fix.  He says thats what happens when they go bad.  When I saked him what goes wrong inside the stator he had no answer.  He said "When the stator goes bad I replace it.  I don't care about the details."  Right now I'm still at a loss on how this one happens I will look more into it later.  

Bill H. 2000 CR250R

but i was right again. i said an electrical problem and you all said no. man i love being right. sorry guys just had to rub it in slightly.  
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