YZ125 Compression test

PK

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#1
I was doing a compression test on my wife's 98 YZ125 and got a reading of 115-120. From what I gather, this is a bit low. What is a good compression on this little motor?

Thanks in advance,
Pat
 
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#2
could be...

My son's KX125 ran great when its compression was around 125-130...
then it really started loosing power when the compression was around 115 and wouldnt run when the compression dropped to 100-110.
Upon tear down we found the ring was really worn and the ring gap had 'grown' to about 1/8 of an inch allowing a lot of blow by. Also, with the ring that worn, there was little ring tension left. But the Cyl and Piston still looked great and were within spec. He has an unreal number of hours on it, I wouldn't even guess how many.
 

PK

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#3
Thanks for the reply, I have a new ring and piston on the way. This weekend we'll tear it apart and see what's what.

Thanks,
Pat
 
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#4
Same thing happend to my 98 YZ 125 right before Dirt Week. The ring actually had a piece missing and lost all compression. New top end kit solved all the problems.
 
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#5
I have a 2000 YZ125 and my compression tested at about 150. My bike runs great. Does anyone know what the service manual recommends that it should be at?
 

marcusgunby

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#6
The comp test isnt a good one for judging when to change a piston.A better bet is to log the number of hours on the piston/ring.A hard rider should look at changing the ring every 5 hours-this is because the YZ has a very large exhaust port and can go through rings quicker than the other brands with smaller ports.A slower rider could extend this to maybe 12hours but i wouldnt go longer than this.They tend to snag the ring and damage the exhaust port bridge.
 
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#7
I just had that happen to my 98 YZ125. Luckily there was no damage the cylinder and a new top end kit was enough to fix the problem. Thanks for the guidlines though marcus. I will definitly keep a ring on hands all the time from now on.