Checked compression tonight - ??

5 replies to this topic
  • jdilpkle

Posted March 18, 2008 - 09:24 PM


1997 XR650L 17" excels, pirelli's, rejetted, pro-comp pipe, uni, airbox mod,..

Motor stock inside. 14,900 original miles. Runs good, uses a little oil after freeway wailing. (Mobil 4T).
Just for fun checked compression tonight w/ motor hot, throttle and choke open:

Auto decompression working = 100psi

Auto decompression NOT working (valve adjuster turned out to about .040" gap) = 145psi.

It figures! - I just ordered and bought a 320mm disk kit yesterday - now i'm thinking I shoulda bought a high compression piston instead. I went for the disk kit because the bike is running fine at this point (prior to test) and I didn't want to open a can of worms if I didn't have to.
Would 145psi be a concern for the moment?

  • cleonard

Posted March 18, 2008 - 09:43 PM


What is your altitude? As sea level 145 is low. At about 2500 to 3000 feet it would be right on.

Did you try putting in some engine oil to see if the number changed? Oil will help seal the rings, but does nothing for the valves.

  • jdilpkle

Posted March 18, 2008 - 10:52 PM


Yeah, I'm about sea level. Bummer. I think I'll keep an eye on it and do a 10.:25 JE piston when I do eventually get into it. No other problems at all.
I was bummed a little - I hate it when I know something is out of spec.
Every time I ride i'll be thinking about it. Oh well, it could have been my last bike, a 4 cylinder 1200 ZRX Kawi! 1 is truly better than 4 in this regard.
I think I'll squirt some oil in the cylinder in the next few days and see specifically what's up. Either way, I'll have to start saving my pennies.
In the mean time, i'll enjoy whoopin' on sportbikes in the twisties like usual, but at a little slower pace due to lower compression. Boy are they lucky - I'll only be 4 corners ahead of them R1's in the tight stuff. :confused:
MoeTardO's Rule!

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  • XR650L_Dave

Posted March 19, 2008 - 07:59 AM


What's the spec say its supposed to be?

15 psi (absolute pressure at stp) x 8.5 = 127.5 (absolute)

127.5 - 15 = 112.5 relative.

145 sounds pretty good, but I don't know what #'s spec.

100psi with the decomp active actually sounds a bit high- are the valves loose?


  • Denn10

Posted March 19, 2008 - 10:59 AM


WEEEE motards rule!!!! WOOOHOOOO only 4 corners ahead LOL too bad. How you liking that mobil 4T? not really the best choice for an air cooled and pretty expensive, i use it on my CBR. Found Autozone is the best price (something like $9.50) and for every $20 us spend you get $5 back so it helps with the cost of that oil

  • resurrection

Posted March 19, 2008 - 04:54 PM


I sure wouldn't panic about a few psi .
I once tested a running sr500 yam. during a pre resurrection and found it only 85psi after a tight valve fix I got 140's and she runs great to this day.

Ya know if it ain't broke don't fix it

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