Mercury compression question

jerkputter

Seaman
Joined
Oct 8, 2007
Messages
53
A few months ago I bought a new Mercury 50 hp power head. I have only run it a few hours and it runs ok. Today for the heck of it I decided to check the compression just to see what a new engine has. The engine was not warm. I got only 90 psi but exactly that on all three cylinders. Is that normal for a new powerhead ? seems a little low to me ? The engine is on a 20 foot pontoon boat and I get around 5000 rpm's out of it whens it warm, about 4800 when first starting out. I have a four blade 10 degree new prop I bought from iboat on it also. There were only two of us on the boat when I was checking the rpm's. The reason I am asking is a small line had came off on the block and it was dumping gas and oil in the cowl area and water. It would not idle until I replaced this line. I am told these lines are bypass lines . Also told they supply fuel mix to the main bearing so if one came off would it mess up the engine ? The line is right under the starter and sort of hard to replace but I did get it back on. I am not sure how long its been off but I did notice the idle problems last week. The engine is oil injected but I have been adding a little oil since the powerhead replace. Thanks for any help on this.
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,786
Possibly your rings haven't seated. I don't think I ever checked compression on a new or newly rebuilt engine but since some engines burn oil (car engines) when new it's possible.

On the tube someone else will have to help you. If it were an oil line certainly having added oil to your gas (premix) would have kept you from doing any damage.

Mark
 

Artie1036

Recruit
Joined
Jul 27, 2014
Messages
3
The compression should be 100+ psi with no more than 15% difference between cylinders.
 

Artie1036

Recruit
Joined
Jul 27, 2014
Messages
3
The compression should be 100+ psi with no more than 15% difference between cylinders.
 

jerkputter

Seaman
Joined
Oct 8, 2007
Messages
53
Drove the boat today for about an hour. Brought it back to my lake house and checked the compression. All three cylinders had 115 psi, exactly the same. I held the throttle wide open with engine hot when checking compression. Got it to 4900 rpm's when I run it so that seems like what it should be. Thanks for the help
 
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