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KX250 - weak midrange

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Got a buddy having a problem with his '03 KX250. This bike is used for hare scrambles. It has an aftermarket FMF pipe, 13/50 gearing, and a flywheel weight. Bike ran good throughout the rpm range. Then he raced the Cinders race here in AZ. Somewhere during the race he split a radiator, ran out of coolant, and overheated the bike. He DNF'ed and found the radiator problem.

Since then the bike is very weak in the mid range. It starts fine and has power up top but is very soft/boggy in the mid. Checked the plug, and the jetting looks rich, but the ground electrode was missing about an 1/8" of it and was irregular shaped on the end. Replaced with brand new plug and there is no change in how it runs. Filter is clean, plug good, not losing any coolant, radiator was fixed before riding again. Any ideas as I'm stumped on this one?

Edited by SS109

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I liked the PC pipe with a 2mm spacer between the pipe and cylinder, in addition to the standard gasket.  Brings the powerband down to a usable RPM for that motor. 

It does sound like the power valve may be sticking.  The overheating may have caused some unusually crusty carbon deposits to form on the valve or in the pocket.  The problem may also be low compression from excessive combustion chamber leakage.

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I ran Pro Circuit pipes with R304 shorty silencers on two of them, a regular Factory sound silencer on one of my KX's.

Sounds to me like he should have pulled the cylinder and inspected the head and cylinder for damage. If antifreeze got into the motor it's &%$#@!ed needs to be rebuilt

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I know the pipe isn't the issue. He was plenty happy with the power the pipe gave him before it overheated.

I'll pass on the info on the power valve/cylinder to him. Hopefully he can get it figured out before race season starts.

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Have you done a compression test?  Running out of coolant in a race sounds like a top end rebuild, or inspection at the very least.

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160-200+ depending on elevation and if any motor works been done.  

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He cooked it. Something is wrong. Probably mechanical damage. Pull the cylinder off and the problem is usually obvious. New piston at a minimum.

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It's probably in a very sad state.

Advise him not to buy a 250F next!

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Just thought I should post an update on this. Surprisingly (after what I found) compression checked out at the low end but still in spec. After all the other stuff was checked, I told him I needed to start tearing in to it to find out what was wrong so off came the jug. Well, you see the problem in the pic below! Even more surprising is that the bottom end lived and made it through four more races and probably 20+ hours of general riding! Cylinder is currently at Millennium getting a 265 big bore.

20170629_083242.jpg

Edited by SS109

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During those 20 hours, where did the skirt pieces go? All made it out?

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It would be very difficult I think for that piston skirt to make it out of the crank and into exhaust.  I would be splitting the bottom end.

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1 hour ago, DylDog said:

It would be very difficult I think for that piston skirt to make it out of the crank and into exhaust.  I would be splitting the bottom end.

Not really.  It would be similar to dropping it into a blender.  Would knock it around until it was pulverized and blow the bits right through.

I would be tempted to run that bottom end if it is tight and runs quiet.

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I guess it depends how the piston breaks apart.  When my skirt blew like that the chunk would have had a hard time making it cleanly through the crank then through the ports. Not out of the question though

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8 hours ago, SS109 said:

Just thought I should post an update on this. Surprisingly (after what I found) compression checked out at the low end but still in spec. After all the other stuff was checked, I told him I needed to start tearing in to it to find out what was wrong so off came the jug. Well, you see the problem in the pic below! Even more surprising is that the bottom end lived and made it through four more races and probably 20+ hours of general riding! Cylinder is currently at Millennium getting a 265 big bore.

20170629_083242.jpg

wow , he got lucky . when mine did that it locked up and split the case

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3 hours ago, kxrob said:

when mine did that it locked up and split the case

Unlucky. How many hours on it when yours failed? What type of piston (cast or forged) and what riding/usage?

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1 hour ago, numroe said:

Unlucky. How many hours on it when yours failed? What type of piston (cast or forged) and what riding/usage?

At a guess I would say 40-50 of my riding ( 25 hours on the hour meter )

And unknown hours from previous owner

it was a vertex (cast) piston , I only ride state forests and tight single track . I do open it up on power line tracks and on our state forest 4wd ( wider and rocky ) tracks

the hours may have been anyone's guess really. Bore was under the factory service limit , rings had 3 x the standard gap and piston was 0.2mm under sized . So from my measurements about 0.25+ piston to bore clearance instead of 0.05 clearance 

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