# Outboard Compression numbers.



## Jeremy_James (Feb 27, 2013)

Where do you guys obtain compression numbers for outboard 2 strokes and 4 strokes.


Cheers.


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## topnative2 (Feb 22, 2009)

call a shop

usually they are iin the 115 range--- more important that they are all have nearly the same reading


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## Brett (Jul 16, 2008)

I have 2 compression gauges. They read about 8 psi different on same outboard.
As long as I'm reading over 100 psi and no 2 cylinders vary by more than 10%, I'm happy.
Expected reading is found in the OEM service manual for your outboard.


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## Vertigo (Jun 3, 2012)

The compression you should see will vary with the make and model of the motor. 100psi would be very low for most normally aspirated motors. I see 155 psi on older Johnsons and Evinrudes, 130 is typical on many Yamahas. Also, some motors will, by design, have different compression in different cylinders from top to bottom, some Yamahas and Johnsons are that way. The best bet is to have the specs on the motor you're checking, otherwise you'll just be wasting your time gathering useless data.


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## topnative2 (Feb 22, 2009)

"  Also, some motors will, by design, have different compression in different cylinders from top to bottom, some Yamahas and Johnsons are that way.  "


Never ever heard of that my education must be lacking.


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## Vertigo (Jun 3, 2012)

Here's a little discussion of the variable ratios for Yamaha outboards:

http://www.yamahaoutboardparts.com/forum2/compression-ratios-th22584.html

Also, be aware that the results of a compression test will vary with temperature, barometric pressure, gauge, technique and phase of the moon. I would generally conclude that if a motor starts easily and runs well, a compression test probably won't tell you much you don't already know. OTOH, if the motor has problems, a compression test can show you where to start looking for a solution.


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## Brett (Jul 16, 2008)

> a compression test probably won't tell you much you don't already know.  OTOH, if the motor has problems, a compression test can show you where to start looking for a solution.


That's why I'm happy with 100 psi as a low reading. If I can get 100 psi with my antique gauges
I'm satisfied the rings/walls/valves/piston/gaskets are still doing their jobs. For a true test of 4 stroke cylinder function
the method I've been using the past few years is the leak down test. Much more thorough diagnostic.

http://en.wikipedia.org/wiki/Leak-down_tester


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## topnative2 (Feb 22, 2009)

Son of a gun!

Thanks, maybe I can make some dinero on that bit of trivia!!!!!!!!!!!!!!!!


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## permitchaser (Aug 26, 2013)

My old '87 Merc 90 did 130 per cylinder and my current Suzuki did the same... Go figure


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## mxbeebop (Mar 22, 2013)

x2 on the leak down test gives a better indication of valve/valve seat condition.


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## Ginclear (Aug 11, 2013)

Thanks , Brett , for the leak down test information . I had never heard about it before your post .
In fact , while I'm thinking about it , thanks for all of your most interesting and informative posts .
Your posts are probably the best reason to follow Microskiff .
I just had my 200 Hour scheduled maintenance done on my 70 2s Yamaha . The compression
numbers , written in sharpie on the block , read , from top to bottom , 130 , 130 , and 120 . I'm 
told that those are good numbers . But could a leak test reveal problems that don't show at all
in a compression test ? Before the maintenance I noticed an intermittent drop in top end speed and RPM 
which I discovered was due to a loose spark plug clip . After I picked the boat up and fished it , the 
motor performed well at first , then dropped in top end performance again . No loose spark plugs , that
had been corrected . I'm wondering if a leak down test would help .


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## Brett (Jul 16, 2008)

Sounds like you have full function in the piston/cylinders.
I'd be looking at electric and fuel/air problems for the loss of power.

My boating "expertise" is earned the hard way.
First I do something stupid with my boat/outboard and break it.
Then I figure out what I did wrong, and learn how to fix it.
Good thing it's only a hobby and not how I earn my living.
I'd be broke several times over!  :-[ ;D


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## Creek Runner (Sep 1, 2011)

Leak down test are only viable options on 4 stroke engines and compression test are no longer even recommend on 4 Stroke Outboards by Yamaha or Mercury. 

A compression test is still the best test for a 2-Stroke engine and should be performed before any repair service is done on a 2 stroke engine. 

130, 130, 120 are good numbers just check it every year when your annual service is done to make sure the gap doesn't widen.


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## Ginclear (Aug 11, 2013)

Brett , thank you for your reply , even a ' hobbyist's " knowledge about outboard engineering surpasses mine .
Creekrunner , I know you are a pro . , so I thank you for your input . I did not know that compression tests 
were not helpful for 4 strokes . Thanks again to both of you for the info and advice .


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