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Author Topic:   How much compression
Einar posted 06-21-2001 11:54 PM ET (US)   Profile for Einar   Send Email to Einar  
I am looking at an 86' Merc 90. How much compression should I expect? Under 300 hours on it.
Bigshot posted 06-22-2001 03:06 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I had a 115 that had around 120lbs. Every guage will give you a different reading so this is the scoop. Remove all plugs and go top to bottom. They should all be within 10% of each other. So if you have 120, 115, 122,118,etc you are cool. If you have 110,110,110,110,90,110, do that low one again to make sure but if it is still low, it is going. Do the test, write them down and let me know.
counterstrike posted 06-22-2001 08:08 PM ET (US)     Profile for counterstrike    

Reality is:

The way to determine if the engine has a mechanical "flaw" is to base the compression on the highest cylinder and ALL should be within 25 % or LESS of the HIGHEST.
Any I.C.E. will run with aprox 75 psi compression as long as ALL cylinders are within the 25 % range.
This rule is for ALL I.C.E.

The thumb rule is:

If the engine has any deviation in compression by 10% dont buy it for the asking price! Putting a small amount of Oil in the cylinders (He'll be here in 20 minutes "Squirt,Squirt" ) will seal the rings tighter to the jug and can cause an incorect reading aka Higher compression on the Day of showing to Buyer !!!

ASE certified master tech

Bigshot posted 06-25-2001 11:31 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Wet tests do not work on 2 strokes!
Steve Leone posted 06-26-2001 07:59 PM ET (US)     Profile for Steve Leone  Send Email to Steve Leone     
It is neccessary to do a "leak down" compression test to know for sure the condition of the cylinder walls,piston,and rings. Factory compression is listed in the manual. Some Mercs were 125lbs. Some were high compression: 150lbs. I agree that attention is warranted for variations of compression between cylinders. The manual also states these variations. I usually use a rule of 15lbs. Always do a compression test cold. Always remove all the spark plugs. Always disable the ignition (kill switch) or ground the spark plugs. If an outboard has been sitting for a long time sometimes the rings will stick. Use a recomended "power tune" and then re-check the compression. Or take it to a shop and pay them to check it out. The last sugestion is the best, believe me! Steve.
counterstrike posted 06-27-2001 10:44 PM ET (US)     Profile for counterstrike    

Steve you are Absolutley correct sir!!


If you dont know what your doing ....ASK.

The dumbest question in the world is the one question that was'nt Asked!!!

counterstrike posted 06-27-2001 10:48 PM ET (US)     Profile for counterstrike    

According to Whaler's records, 3A4684 was sold to one of our Engineers as a
stripped hull 16'7" model and was picked up at the Rockland, Mass factory
on 6/30/71. Stripped, meaning there was nothing installed at the factory.

Yes, Whaler's records indicate that this serial number was on consignment
as a bare/stripped hull to our head engineer at the time, Bob Dougherty. It
does not mean that the boat was completed, yet.
I'm sure that the boat was finally sold (private sale) as a complete boat,
however, Whaler does not have records for this...sorry.

Great pictures...Good luck with the restoration.

Chuck Bennett
Customer Care Rep.

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