Low compression?

Mnemonic

Petty Officer 1st Class
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Nov 3, 2004
Messages
285
Just did a compression test, and the battery wasnt fully cranking the motor... cyl #2 and #4 even with 2 cranks went up to 125... <br /><br />But #3 went only to 45... thats WAY low... and I dont know if its accurate because the battery was too low and fully cranking, but now im freaking out about this cause i just bought it! <br /><br />How do you fix low comression if that is the problem?
 

Barnacle_Bill

Admiral
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Feb 8, 2004
Messages
6,469
Re: Low compression?

You need to recheck it with a fully charged battery before you get worried.
 

Trent

Captain
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Nov 17, 2001
Messages
3,333
Re: Low compression?

How do you fix low compression??? Normally a engine rebuild. :( <br />You could get lucky and try to decarb the engine.
 

Moody Blue

Captain
Joined
May 24, 2004
Messages
3,136
Re: Low compression?

First things first. Do a proper compression test before you start planning a rebuild. Don't worry about what you don't know.
 

JB

Honorary Moderator Emeritus
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Mar 25, 2001
Messages
45,907
Re: Low compression?

Even firster things before that:<br /><br />What engine do you have, Mnemonic?
 

Mnemonic

Petty Officer 1st Class
Joined
Nov 3, 2004
Messages
285
Re: Low compression?

1980 Mercury 90HP inline 6...<br /><br />The problem wasnt the battery, it was the crappy old cables from my last motor... the battery is brand new, just bought new cables and will retest tonight.
 

Mnemonic

Petty Officer 1st Class
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Nov 3, 2004
Messages
285
Re: Low compression?

Anyone know how to remove the top cowl and the lower part of the cowl to reach cyl 1 and 6?
 

Mnemonic

Petty Officer 1st Class
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Nov 3, 2004
Messages
285
Re: Low compression?

OK - Didnt have time to figure out removing the top cowl - but i did test 4 out of 6 that were accessable.<br /><br />#1 - Didnt Test<br /><br />#2 - 150<br /><br />#3 - 75 !!!!!!! My nightmare come true!<br /><br />#4 - 150<br /><br />#5 - 135<br /><br />#6 - Didnt test<br /><br />This is showing that #3 is half of the norm! I just bought this damn motor and put over $500 in controls and cables and crap to find out that it has low compression! Im going to get it running tomorrow, I will try a decarb, but dont you have to spray decarb while at WOT? this motor is sitting on a stand, i dont want to do WOT when the motor isnt in water.
 

Clams Canino

Commander
Joined
Jan 10, 2004
Messages
2,179
Re: Low compression?

You can do it at a fast idle. I suspect #3 is toasted though. Quite common failure.<br /><br />-W
 

Mnemonic

Petty Officer 1st Class
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Nov 3, 2004
Messages
285
Re: Low compression?

Clams - according to the manual it says this could be due to the piston rings sticking. Its first attempt to remedy this problem says to put a tablespoon of oul in the problematic cyl and try again.<br /><br />Also, this motor has not been started for 3 years...<br /><br />What if that doesnt fix the prob tho? I am not a good enough mechanic to do a powerhead rebuild and I dont want to pay anyone any more to do this!
 

dynapac

Cadet
Joined
Apr 14, 2005
Messages
27
Re: Low compression?

Use a liberal amount of fogging spray then the tablespoon of oil. Gently turn over the motor so you can spray the entire cylinder wall.<br /><br />I think you can do the rebuild. Take photos of everything before you touch the engine. This will give you a perfect representation of how things were prior to disassembly. Organize all tools. Buy a case of beer and go chew the fat with your local marine mechanic. Take photos during every step. Ask his advice and pointers on the job. Don't scimp on parts. Replace all gaskets, bushings, bearings, etc. Get a TORQUE wrench. Get a Manual. Take your time. <br />If your testing to date proves your theory correct, you haven't lost anything by tearing down the motor. <br /><br />I ran into the same issue on my 1987 Merc 60 3 cylinder. Never owned a boat before, took me a little over three months to repair the motor (I had to replace the midsection, lower unit, rebuild the carbs, rings sticking on #2, bad starter, replace power trim/tilt pump, solenoids and tachometer, etc, etc. Never owned a boat before, just a compulsive do-it-yourselfer) <br /><br /> http://www.themotorbookstore.com/marinemanuals.html <br /><br />I just figured out yesterday on how to adjust my low idle, then adjust the jets on the carbs. Runs like a dream (until next time out!!!)<br /><br />I have found this board to be a wealth of knowledge. I have spent countless hours reading many posts. Most everything you need to know has been discussed here.
 

Mnemonic

Petty Officer 1st Class
Joined
Nov 3, 2004
Messages
285
Re: Low compression?

Originally posted by dynapac:<br /> Use a liberal amount of fogging spray then the tablespoon of oil. Gently turn over the motor so you can spray the entire cylinder wall.<br /><br />I think you can do the rebuild. Take photos of everything before you touch the engine. This will give you a perfect representation of how things were prior to disassembly. Organize all tools. Buy a case of beer and go chew the fat with your local marine mechanic. Take photos during every step. Ask his advice and pointers on the job. Don't scimp on parts. Replace all gaskets, bushings, bearings, etc. Get a TORQUE wrench. Get a Manual. Take your time. <br />If your testing to date proves your theory correct, you haven't lost anything by tearing down the motor. <br /><br />I ran into the same issue on my 1987 Merc 60 3 cylinder. Never owned a boat before, took me a little over three months to repair the motor (I had to replace the midsection, lower unit, rebuild the carbs, rings sticking on #2, bad starter, replace power trim/tilt pump, solenoids and tachometer, etc, etc. Never owned a boat before, just a compulsive do-it-yourselfer) <br /><br /> http://www.themotorbookstore.com/marinemanuals.html <br /><br />I just figured out yesterday on how to adjust my low idle, then adjust the jets on the carbs. Runs like a dream (until next time out!!!)<br /><br />I have found this board to be a wealth of knowledge. I have spent countless hours reading many posts. Most everything you need to know has been discussed here.
Thanks for the info... when i spray the fogging oil in the cyl and turn it over and get the whole wall of the cyl should i start the motor? <br /><br />Or just do another compression test?
 

dynapac

Cadet
Joined
Apr 14, 2005
Messages
27
Re: Low compression?

I think it would depend on what you are trying to accomplish. I assume it is to free a stuck/rusted ring. I would turn it over a few times, let it sit a couple of days to let the oil penetrate. Refog then test for compression.
 

Clams Canino

Commander
Joined
Jan 10, 2004
Messages
2,179
Re: Low compression?

I'd use Mercury Power Tune - to unstick. Let it soak in there a while.<br /><br />-W
 
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