# Compression test knowledge



## Captdroot (Jan 18, 2012)

1) I've been told a compression test on warm/hot engines is more valuable that a cold compression test. Many folks agree? Disagree?

2) The most important thing about a compression test is that *ALL cylinders are* *within 5lb* of each other? Agree or disagree? Comment?

3) What are some *good* compression numbers for a used Yamaha 115 HP *2* stroke?

4) What are some* good* compression numbers for a used Yamaha 115 HP *4* stroke?

5) How about *good* numbers if it said to be *recently* rebuilt, for both 2 and 4 strokes

Looking forward to some feed back. Thanks!


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## Bustedknuckle (Feb 11, 2009)

1) compression test warm true especially on 2 strokes.
2) all numbers should be w/n 10-15% of eachother to my knowledge.
3-5) refer to my 2nd response.


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## BIGRIGZ (Oct 2, 2007)

IMO, compression tests on boat motors should be done with all spark plugs removed, throttle wide open, ground all of the plug wires, manually jump the starter motor with battery cables to override the neutral start switch. Read the compression gauge.

no more than 15% variance, I prefer <10%.


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## drifterfisher (Oct 9, 2009)

A cold and warm test are very informative.I like to see less than 2% difference between those,and 5% or less between cylinders.I do not know specifics for the engines you asked about.


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## markw4321 (Oct 4, 2007)

I just completed a test on two Yamaha f115 4strokes with 350 hours. My readings averaged 205 psi on all cylinders. I searched for what was "normal" for that engine and did not find anything other than anecdotal info suggesting that as long as you were above around 140 psi your compression was "ok".

I used to own a 13 year old 1996 Yamaha 150c 2 stroke with around 1000 hours, it averaged 110 psi and ran fine when I sold it in 2009. I also remember that 2 stroke compression is normally lower than 4 stroke and that the 110 number was "ok" for that motor and age.


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## Captdroot (Jan 18, 2012)

*Thanks and keep the info coming*

Have not owned a boat for over 5yrs. Health took over our lives. I have not done a compression test for around 9yrs. All this info is valuable. I live east of Pensacola/Destin area and I'm about to take some cash and may bring home a boat that is near Tallahassee. My knowledge of boats and test rides is extensive, but my memory of compression tests has faded. Four Strokes are what I'm really looking at, but a smaller (115-140HP) Yamaha 2 stroke might also work. This "rebuilt" thing is always a red flag to me. Rebuilt should produce great compression numbers, right?

Also, I have been "told" that one rig only has *188hrs on it*. I was "told" that information came from a computer diagnostic test??? I would gladly pay a knowledgeable mechanic, but $100 for a 6-8hr excursion, it is just not worth it.


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