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Low compression due to gas in oil?

  • Thread starter Thread starter 79-GS550-L
  • Start date Start date
7

79-GS550-L

Guest
Hi everyone,

I just checked my compression and I'm getting around 100-110 psi cold and maybe 110-120 psi warm. All 4 cylinders are always within around 10 psi of each other, and pouring a little oil into them doesn't change the value by more than 1-2 psi. I just finished replacing most of the valve shims, they're all within spec now. Are those numbers ok? I've read conflicting acceptable ranges in the forums. It runs nicely with new plugs but fouls them very quickly (but that's because the carbs are adjusted wrong). I do know that I have gas mixed in with my oil (bad petcock), and I've heard that that can cause low compression readings. I plan to change the oil before riding it anymore. If that's the reason for the low readings, will fix itself after the oil change?
 
One thing that I found when checking the compression is to have the throttle open all the way.

With the throttle closed, I was making around 115 psi, but with it open, I was getting it closer to the factory rating of 150psi per barrel on my 1979 GS1000. These were all dry numbers. I didn't bother testing with oil as I was satisfied with the results.
 
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Assuming that your test is being done correctly, 100 psi is about minimum. The important thing however is that they are all about the same which yours are.

If when warm you're somewhere around 110 - 120 it isn't brand new, but its good enough. If it runs good, I'd ride it and enjoy.

Having gas in the oil isn't the best of course. But it sounds to me like you are good to go after you fix that and get your carbs figured out.
 
Never the twain shall meet.

Never the twain shall meet.

Mr. 79-GS550-L,

My Suzuki shop manual says you're supposed to warm up the engine before you test the compression. But I would NOT run the engine with gas in the oil. Change it first.

Thank you for your indulgence,

BassCliff
 
thanks for the input everyone. I was doing the test with throttle wide open, and I warmed up the engine a *little* bit even though it had gas in the oil, figuring I'd change it as soon as I got my new petcock installed... hopefully I didn't wear anything idling for that short amount of time. It is good to know that the numbers are not way off. One thing that still bothers me though is that it does not rise quickly... it takes maybe 6 revolutions before the numbers peak. I've read it should be 2-3. What affects that, and what does it mean?
 
Five or six revolutions is not a problem and doesn't mean that anything is wrong with your bike. Just try to do each cylinder the same of course.
 
SOOOOOOOOOOOOOOOOOOOOOOOOOO Tired of telling the forum members this......

PUT AN INLINE FUEL STOP SWITCH ON YOUR MAIN FUEL LINE and turn it to "off" when not using the bike, when you want to go ride turn it to "on"

NO MORE FUEL IN OIL.
Phil.
 
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