YZ125 and YZ250 Compression Testing

kx254f
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kx254f
Edited Date/Time 6/14/2012 9:07pm
I'm looking for the "safe psi range" for both the 2012 YZ125 and 2012 YZ250 compression. I'd like to base my rebuilds on how the compression reads if that's a smart thing to do.

Any info/insight is greatly appreciated. Thanks!
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DPR250R
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DPR250R 6/13/2012 6:33pm
6/13/2012 6:33pm
Are they both new?

Do a compression test after breaking them in so you have an idea of what it should be.

Use that number as a comparison to values as you put hours on the motor.

I remember reading the top end should be done after a 20% loss in PSI is found. That seems a little steep to me though.

Also.... pistons can crack and break apart. An hour meter would be a good idea cause you would not want to leave the same one in there for too long.

220/221... whatever it takes.

RichYox
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RichYox 6/13/2012 6:37pm
6/13/2012 6:37pm
Just did my YZ125,,,, fresh top end made 165 PSI.
kx254f
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kx254f 6/13/2012 6:46pm
6/13/2012 6:46pm
DPR250R wrote:
Are they both new? Do a compression test after breaking them in so you have an idea of what it should be. Use that number as...
Are they both new?

Do a compression test after breaking them in so you have an idea of what it should be.

Use that number as a comparison to values as you put hours on the motor.

I remember reading the top end should be done after a 20% loss in PSI is found. That seems a little steep to me though.

Also.... pistons can crack and break apart. An hour meter would be a good idea cause you would not want to leave the same one in there for too long.

220/221... whatever it takes.

Got the hour meters on them at 20 hours now. Compression and power feels good on both. We run AMSOIL and feel safe changing every 30 hours with that.

I've read 165-180 psi is a "fresh" top-end range for the 125. 120-130psi is time for a rebuild. I'll see how that matches up with the hour meter for the 125. Still researching the 250.

Thanks for the input!
Jim Morrison
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Jim Morrison 6/13/2012 8:33pm
6/13/2012 8:33pm
Compression readings are not a safe way to judge a top ends condition. I have had bikes go 60-80 hours without a ring change and show no compression drop that was noticeable with a gauge. The big risk is that an engine with a fatigued piston on the verge of fracture can still have acceptable compression because all you really are measuring is what is leaking by the rings and the piston has little effect on that. However, for a general rule mine were usually in the 175-185 psi range for 2 smokes.

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floridaflash
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floridaflash 6/14/2012 1:33pm
6/14/2012 1:33pm
On the 125, if you are a serious racer and ride hard new top end 10-12 hours, on the 250 20 - 22 hours, that is what I have used for years with no proplems, also depending on how you are mixing your oil you will get about 35- 50 hours out of your crank. I would run no leaner than 32;1
xnationalwrench 6/14/2012 3:57pm
6/14/2012 3:57pm
Another thing to realize is different guages read different ("A" guage can read 160 psi, while "B" could read 170psi)
project racer 6/14/2012 9:07pm
6/14/2012 9:07pm
xnationalwrench wrote:
Another thing to realize is different guages read different ("A" guage can read 160 psi, while "B" could read 170psi)
True. Some of the automotive type guages have a larger diameter hose on them ,also. Kawasaki makes one of the better compression guages for motorcycles with a small diameter hose, and it is virtually leak proof. Using the kickstarter can also lead to inconsistent readings.

It's a bit of a pain in the ass, but install the guage, put the bike in second or third gear,and run with it like you would for bump starting, for about 40-50 feet,with the throttle fully open.

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