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Compression Needed for Good Operation 1976 850 85 hp

M

Michael Albert

New member
My 1976 Mercury 850 Outboard (85HP) has given me years of great service. This year I have been unable to get the motor to run consistently, and wonder if it could be due to the relatively low compression readings of 110 to 115PSI per cylinder.

Is there a minimum anyone is aware of?
 
Re: Compression Needed for Good Operation

That's pretty low for that motor--about 140 is good. The rings are probably stuck in the grooves, which can sometimes be cured by using Merc power head tune juice. If not, it's tear down and re-ring time.

Jeff
 
Re: Compression Needed for Good Operation 85HP Merc

Thanks for your comment that 140PSI is good compression. Till this year it had always been over 145psi. Are you telling me that I cannot make this motor run smoothly at 110PSI compression no matter what? I expect the power to be down, but can I make it operate smoothly and have the full rpm range if all else is correct?

Thanks again!
Michael Albert
 
Re: Compression Needed for Good Operation

Traditional wisdom has said about 10-15% low was the max tolerance.

OMC/BRP revised their position a couple years back to 20% citing "un-necessary" rebuilds.

If 140 was (spec) for your model, a 20% drop would be 112, which would be the most "generous" lower end recommendation out there from those who build these things.

Try the "PowerTune" as Jeff suggests, it's only 5 bucks (available at any Merc dealer) - if it's simply carboned rings that will raise your compression to an acceptable level.

If not, it really is time for new rings...
 
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