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compression

L

lstring

Contributing Member
I have a 1977 70hp Evinrude three cylinder with 90 psi on each cylinder, is this a sign of needing
a rebuild ?. it seems to be low to me.
 
There are a variety of things that can cause low compression readings including testing in the water with choke on or no throttle opening. Another can be a motor that has been stored awhile. Thirdly, a slow cranking speed. Fourthly an inaccurate gauge. My gauge reads low, about 10 pounds low. Got oil in it once and it was never the same. Its been with me for 45 years. I know the gauge. Today I tested a 2000 8hp Johnson. Wife Raquel found it in the pawn shop. Maybe run 5 cans of gas in its entire life. Tested 82 and 83 psi. I calculate it sat awhile too. The motor hasnt been run for a couple years. Long story short. Your motor may be just fine.
Another effective check is to look for signs of overheating. In severe cases, melted wires near the head/heads. Discolored paint on the head/heads is generally bad news. Multi carb motors can lean only one or two cylinders if a carb problem has gone ignored, the result will be scoring in that particular cylinder with resultant low compression.
 
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As Tim states... compression relates to cranking speed. A slow cranking engine will result in a low psi reading. The fact that all cylinders are even in compression is a good sign. Also as Tim states... double check with another gauge.
 
I think I'm gonna get a new gauge like yours, Racer, that shows more accurate and optimistic numbers. What kind and model is it?
 
Huh? Can you answer my question, yes or no? If you bought a new gauge what would you buy? Joe, you crack me up. An accurate gauge is a must, but my gauge "accurately" reads 10 psi low. So I'm an idiot and a smart ass, just like my wife says.
 
timguy said:
Huh? Can you answer my question, yes or no? If you bought a new gauge what would you buy? Joe, you crack me up. An accurate gauge is a must, but my gauge "accurately" reads 10 psi low. So I'm an idiot and a smart ass, just like my wife says.
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Hmmm... Tim, I lost my wife of 49 years, it will be 8 years ago this coming October at her age 79. I have many fond memories of her. However... at some point in her 79 years on this planet, I feel quite strongly that she and your wife must have spent a good deal of time together somewhere! :)
 
Could be, Joe. Probably 2 of the greatest women ever. (Our wives). My first wife said that no woman could ever put up with me more than 13 years. Well now 19 with Raquel, still having a great time look forward to each day.
So, Racer has a good compression gauge, I like those numbers. Time for a new gauge for me.
 
timguy said:
An accurate gauge is a must, but my gauge "accurately" reads 10 psi low.
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So Tim, what you have there is a precision gauge. It reads precisely 10PSI below the correct value every time. Unfortunately, it's just not accurate. ;)

Precision - Closeness of multiple measurements of the same thing

Accuracy - Closeness to the true value.

My bathroom scale weighs me precisely every time, I just wish it was a lot less accurate. It tells me my true weight, and I wish it would lie to me a little :)

KJ
 
Okay, now I understand. Now I know the difference.........not much as long as you can consistently make a "reasonable" evaluation. It kinda goes along with knowing just how much your wrong and compensating for it with enough BS. Got it!
 
timguy said:
......It kinda goes along with knowing just how much your wrong and compensating for it......
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Yep. On the golf course I nearly always pull my drives slightly to the left. So, I just line up a little to the right to compensate. Of course, that is until my swing somehow straightens out and now I push it to the right. And that is when my precision goes right out the window!!! :eek:
 
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