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Author Topic: '02 KX125 engine dies at high rpms  (Read 9601 times)

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Offline Shadow

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'02 KX125 engine dies at high rpms
« on: March 01, 2011, 06:29:08 AM »
Went for a ride first time after a top end rebuild. Bike starts up and idles fine but every time I get into high rpms and then release the throttle, the engine dies instantly. No bogging, no cutting or skipping or anything like that. It just dies without any warning.

I'm seriously at a loss as to what might cause it.
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Offline ford832

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Re: '02 KX125 engine dies at high rpms
« Reply #1 on: March 01, 2011, 10:24:50 AM »
If you continue to hold it at a high rpm does it keep running fine or start to slow down?If you do the same thing at about 3/4 throttle will it do the same thing?Does it immediately start back up again or does it take a few kicks?
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Offline TMKIWI

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Re: '02 KX125 engine dies at high rpms
« Reply #2 on: March 01, 2011, 11:09:03 AM »
Have you run it in.
Sounds a bit like a seize  :-X
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Offline MyckMcClung

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Re: '02 KX125 engine dies at high rpms
« Reply #3 on: March 01, 2011, 06:28:18 PM »
check the spark plug wire
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Offline Shadow

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Re: '02 KX125 engine dies at high rpms
« Reply #4 on: March 06, 2011, 07:47:05 AM »
Sorry for a late reply. Didn't have a chance to take a look at the bike until today.

Somehow the bike had been cured miraculously while it was sitting at the garage. Started it up to check what ford said, at first it bogged a bit like it was too rich but then it opened up, and started revving pretty much perfectly and didn't die. This was while the bike was in neutral though. With spike tires on it and the whole area near the garage being asphalt, I can't really test in gear there.

TMKIWI: The issue actually came up during the run in. Never knew that kind of behavior is a symptom of an engine seize. I always thought it's more like bike dying completely and won't even start again.

McClung: Checked, no issues there.
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Offline Shadow

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Re: '02 KX125 engine dies at high rpms
« Reply #5 on: May 31, 2011, 05:36:38 AM »
Update: Managed to test ride the bike properly only now, and the problem still persists.

From 3rd gear and up, when I let off the throttle on high-to-top rpms the bike shuts down. I can keep the rpms up no problem but every time there's a drop in the rpms the bike dies.
Can't hear any odd sounds from the engine, bike starts well, picks up rpms well, low to mid-high rpm range works normally and the problem is not present when the bike is in neutral, only when the engine is under load. Air screw did not seem to have any effect.
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Offline MyckMcClung

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Re: '02 KX125 engine dies at high rpms
« Reply #6 on: May 31, 2011, 06:26:08 AM »
Check the power valve connecting rod. it may have been installed improperly.
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If a pair of 2" brass balls isn't working, I doubt that the 3" model will make much difference.

Offline Shadow

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Re: '02 KX125 engine dies at high rpms
« Reply #7 on: June 01, 2011, 07:50:06 AM »
Do you mean the governor shaft lever + the idle gear lever and main shaft gear alignment?

Pardon me, but I have no previous experience with KIPS or power valves and the repair manual I've got doesn't list a part named "power valve connecting rod".
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Offline MyckMcClung

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Re: '02 KX125 engine dies at high rpms
« Reply #8 on: June 01, 2011, 07:52:41 AM »
yeah that's what I meant, just using laymens' terms
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If a pair of 2" brass balls isn't working, I doubt that the 3" model will make much difference.

Offline Shadow

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Re: '02 KX125 engine dies at high rpms
« Reply #9 on: June 01, 2011, 08:01:27 AM »
Alright, just making sure, 'cause I don't want to strip the whole KIPS apart if I don't have to.

Checked that and it MAY have been misaligned. Not too sure since I saw the mark on the main shaft gear only after I took it off, but at least now it is aligned without a doubt. Will head to the track tomorrow to test the bike again.

Makes troubleshooting a bit tiresome when the bike is stored some 10km from my house and I can't test ride there, so I need to drive to a nearby track every time I want to ride the thing a bit. Fortunately that's only a 10 min drive.
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Offline MyckMcClung

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Re: '02 KX125 engine dies at high rpms
« Reply #10 on: June 01, 2011, 05:27:20 PM »
10 minutes?? wow, the closest place for me to ride is 45
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If a pair of 2" brass balls isn't working, I doubt that the 3" model will make much difference.

Offline Shadow

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Re: '02 KX125 engine dies at high rpms
« Reply #11 on: June 02, 2011, 05:53:47 AM »
No luck. Still dies in high rpms.
Remember when I said that it doesn't happen in neutral? Well screw that. Today it died in neutral, not even sure if I let off the gas when it died.  I'm completely confused at this point.
Starts back up on first kick after it's died. No issues with compression. Only other problem I have is that the clutch doesn't disengage properly but that shouldn't have anything to do with this.

Could the problem be with the main jet in the carb and it being clogged or something?


Haha, yeah, I've been fortunate enough to have always lived near a track. And the riding cost for a season is 30 euros. :D
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Offline SachsGS

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Re: '02 KX125 engine dies at high rpms
« Reply #12 on: June 02, 2011, 07:42:20 AM »
Give the carb a good cleaning and focus on the idle and pilot circuits. ;D
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Offline citabjockey

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Re: '02 KX125 engine dies at high rpms
« Reply #13 on: June 02, 2011, 02:31:46 PM »
So the failure mode is when you rev it, you can hold the revs with a bit of throttle but when you chop the throttle the engine slows and then dies -- even when you are re-opening the throttle? When it does this, take out the spark plug. Is it wet?
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Offline Shadow

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Re: '02 KX125 engine dies at high rpms
« Reply #14 on: June 02, 2011, 03:54:56 PM »
Yeah, reopening the throttle does not help.

Update: Alright, the carb is now squeaky clean. What little I can test the bike at the storage depot, the bike picked up rpms frighteningly well and showed no signs of dying. I could keep the throttle wide open without problems. That's in neutral though, so how it works at a track still remains to be seen.
« Last Edit: June 03, 2011, 07:31:23 AM by Shadow »
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