stock 125 compression readings (PSI)?


xERICx

Member
Jan 31, 2003
17
0
I finally did my first top end.
The compression before the top end was 167psi.

After breaking in the top end and riding it under 1/2 throttle for about 20 minutes I get a reading of 180psi.

What range should a stock 125 be?
Bike is a 2001 KX125.

Also, after a rebuild when is the best time to do a compression test? Does it matter if I do one at 20 minutes or will the top end need to be broken in further to get a more accurate reading.


This has been buggin me for a couple days now.
I thought that this bike had a modified head or cylinder (higher compression) but the guy who sold me the bike said he ran pump gas. I do know that the bike was ported, and the company that did it has machined there name into the side
I talked to the company that ported it and they could not tell me anything about the motor, they just knew that they ported it.
 

darringer

LIFETIME SPONSOR
Dec 2, 2001
1,029
0
A compression check is a poor way to determine top end condition. Better to check ring end gap. Your compression figures are in the ballpark for a fresh 125. Also, the porting of a cylinder does not necessarily raise static compression.
 

rairden

Member
Jun 3, 2003
74
0
That's good advice. I just got a 2003 RM125 (2nd bike). My first one is a '01 YZ250 that I still have. Anyways I've only ridden this RM for 10 minutes(just got it 2 days ago). I took a compression test and it's a an even 200 psi. I've heard it needs to be around 155-195. I love this RM and it feels like the 250 in power.
My question is how long will the ring(s) last? I know you'll probably say 20-30 hours or so. But I ride 90% of the time on a track in high rpm's. And ride once a week. How long will it last. A full year? Half year?
 

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