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Author Topic:   Engine Compression
Ray posted 06-11-2002 12:10 PM ET (US)   Profile for Ray   Send Email to Ray  
Does anybody have the compression specs on a 1986 225 hp Johnson outboard and also a 1988 225 hp Johnson outboard?
Tom W Clark posted 06-11-2002 12:23 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
No such thing really. It depends on several factors. See:
Ray posted 06-13-2002 08:52 AM ET (US)     Profile for Ray  Send Email to Ray     
Thanks for the reply Tom. I get a very consistant check within less than 5# per cyl. but the readings are between 85 and 90#. This seems low although the engine ran great last fall before storage. I squirted some oil into each cyl. and the readings only jumped about 5# each. I am told that those years were built with low compression to compensate for the low octane fuels. Does all of this make sense?
Tom W Clark posted 06-13-2002 10:23 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

What you may have been told is that those engines were built with a relatively low compression ratio. This is different than low compression, though I have to assume that a motor with a relatively low compression ratio will not produce as high a compression reading as measured by a compression gauge in psi.

Sounds like your motor is fine in this regard.

T Party posted 06-13-2002 11:53 AM ET (US)     Profile for T Party    
I have been told by my mechanic that I have low compression in two cylinders (the lower ones on my 1991 Johnson 88SPL). He told me not to run the boat wide open anymore, take it easy and I'll get through the season. Questions:
Can anything be done about low compression or is the end drawing nigh for this engine?

If anything can be done, is it worth it? It's been an extremely reliable motor - no problems other than routine maintenance.

Is this why we should use ring-free or similar additives?

Steve Leone posted 06-16-2002 01:59 AM ET (US)     Profile for Steve Leone  Send Email to Steve Leone     
the v-4 o.m.c. seem to be notorious for those two cylinders. low compression in those cylinders could indicate some real serious problems going on. if you intend to rebuild the powerhead you should stop running it. running it will only agravate the problem and there is a chance you might spit a ring or worse. de-carbonizing the engine does help. as an outboard carbons up the carbon tends to harden with the "hot/cold" process. sometimes your compression will skyrocket ( i`ve seen them at 190 lbs psi and more) when this occurs. sometimes a piece of that very same carbon will break off. sometimes it goes out the exhaust, sometimes not. when it does not it can and will get lodged between the cylinder wall and the piston and/or rings. carbon is hard (the stuff of diamonds). it will cut vertical groove in the cylinder. thats when your compressin will leak by. a yearly bout with engine tuner or de-carbonizer will help prevent this from occuring. i have a rebuilt powerhead for your outboard if you are interested, steve out.
Bigshot posted 06-17-2002 11:41 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Run a can of techron in a 1/2 gallon of gas(& oil) and decarbonize the engine. This should free the rings up. You can also buy the powertune stuff and follow directions. To do the techron just run it until she uses it all and then let it sit overnight. Reconnect fuel supply and go. Then do compression readings again and compare to before(so do before).
T Party posted 06-20-2002 03:24 PM ET (US)     Profile for T Party    
Thanks, guys! I'm going to try Bigshot's suggestion (it can't hurt, right?) and if it doesn't help (which I kinda expect, since I've put about 15 hours on the engine since I detected the problem), I may be emailing Steve about that rebuilt powerhead.

Thanks again.

Bigshot posted 06-20-2002 03:36 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Again this was recommended at Yamhah training school to decarb an engine(also cleans out carbs).

Take a bottle of techron and pour itinto a 1/2 gallon of gas. Run engine at high idle or so until gas is used up but engine does not run out of gas(you want it to still be in carbs). Shut it down and sit overnight. rehook gas supply and run engine like normal. Make sure you add oil to gas if VRO is disconnected! Make sure you take comp test before and after to see if anything changed. this will usually unstick rings. If cyl walls are already scored...oh well. Being your engine is on both lower cyls(which the lower carb feeds), Your lower carb may be running too lean.....or too rich which would then carbon up the rings. If compression does come back to within 10% or so of the top just raised the dead. If it does not could run another 10 years or 10 minutes.

Post your comp readings as soon as possible. I am curious to see what they are. if you do not have a comp tester.....get one. Most Pep Boys etc have them(screw in type) for about $25. Will come in handy. I do comp tests at the beginning and end of every season so i can monitor engine wear. then again I don't really have seasons down here,but I used to in NJ.

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