Minimum compression?

[shoestringmariner]

What is the minimum compression you will accept when buying a motor?

[david-bartlett]

Trick question. Need to know what motor. Acceptable values will vary.

[thumper]

Higher horse power motors will require higher compression. Mercs also have higher compression than the johnson evinrudes etc. 60 psi is ok on low hp omc. 80psi or better on 10 hp omc or higher is best. Just what I have been told not really a rule of thumb.

[shoestringmariner]

Thanks. I’m seeking out a 10hp motor, but don’t want to waste my money dragging home something that I can’t rebuild.

Thinking of a Pre 60’s Evinrude or Sea-Horse.

[Phil Budne]

80+ would be best even on the low hp ones. A 3 hp 50s twin will idle amazingly low with 80+
I will usually take one in the 70+ psi range especially if it’s very nice cosmetically. I find that flattening the head and deck and using a new head gasket will often gain me 5-10psi on the comp. reading.
Also if the rings are stuck it will comp. better when they are free. I have a 1952 Johnson 3 hp that I don’t dare try to take apart because it’s been in salt a lot, but it has 88 psi comp. and runs great so I don’t really need to take it apart.

http://www.omc-boats.org
http://www.aerocraft-boats.org

[fleetwin]

Be sure to check the gear lube before buying as well. No gear lube, or a gearcase full of water spells trouble. These old 10hps were prone to wrist pin wear as well, so rock the flywheel back and forth gently to check for sloppy wrist pins.

[bluehaze]

The type and model is what matters in an outboard’s compression readings. Not necessarily the hp or size.
Some of the 9.5’s had 65-75 lbs when new. The late 80’s 200 hp looper motors only had about 85-95 lbs when new. Some outboard designs will read well into the mid hundreds.
My Merc MK5 5hp reads about 120lbs, while my 1961 Evinrude 5.5 reads around 85lbs.

[johnny-infl]

I know this is an old thread, but, I have the same issue…….
Just traded my brother for a 1969 9.5hp Evinrude that has been sitting for over 10 years.
It has spark, turns over "freely" with the pull start . . .
but, it only has 40psi compression. I am thinking of equaling out the cylinders and put
in some sea foam, one ounce twice a day and see if that loosens up the rings to get better numbers.
Is this the way to go ? or use another fluid like ATF/Acetone mix, PB Blaster, etc.
I really don’t want to start taking this motor apart.
I will build a motor stand this week so I can put it in water and try to get it running.
I read the other posts about 5oz oil and sea foam to a gallon of gas . . . will try that too if it runs.
I would really like to see 80+psi.

[chris-p]

As stated, the 9.5 are low compression motors. They wont have high compression readings. All the ones I have worked on were around 65 to 85 or so.

Check the head gasket, might be damaged between cylinders, giving overall low reading in both. Check to see if rings are stuck, if so, seafoam or OMC engine tuner will help.

[crosbyman]

If I was you I would try to revive it .. I found 0 lbs compression totally stuck engine and it came back to life after weeks of soaking and loosing up the rings

keep at it !! ( seach PRISTINE 9.5 )

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