Compression?

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tipitina

Petty Officer 2nd Class
Joined
Apr 15, 2004
Messages
133
I posted my boat up on ebay, and said it had good compression. When I tested the compression, it came out between 70 and 85 on all 6 cylinders, and the mechanic said that should be okay. Now, someone on ebay tells me this is bad compression, and the motor shouldn't even run with this compression. I did the test cold, and the lower unit wasn't on when I did it. Should this matter, is it true this is bad compression? I don't want to mislead anyone. The motor is a 1976 mercury 1500, 150 hp.
 
O

oregonfish

Seaman
Joined
Nov 18, 2004
Messages
53
Re: Compression?

IMHO, 100 or better is ok. The difference between the cyl's is the iffy part though.
 
C

Coolmaker

Petty Officer 2nd Class
Joined
Mar 15, 2005
Messages
134
Re: Compression?

Try a different compression gauge. Should be bo more than 15% difference in cylinders.<br />Good Luck
 
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tipitina

Petty Officer 2nd Class
Joined
Apr 15, 2004
Messages
133
Re: Compression?

So could the lower unit being off affect the test? (Making it easier to turn maybe?) Does the fact that the motor is cold affect the test?
 
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stougas

Cadet
Joined
Jan 14, 2002
Messages
17
Re: Compression?

The lower unit doesnt affect a compression test on an outboard. Your compression should read between 120-125 on all cylinders. The rule of thumb is if they are 5-10 lbs difference between cylinders you should be ok. If one cylinder is 120 and one is 65 then you have a problem. If they are all equal within reason you are ok. If it runs fine dont worry about it. You posted the facts what more can you do!<br /><br />Good luck
 
Clams Canino

Clams Canino

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

These numbers are bad for a 1500, but I bet it's the guage or it's real dry from sitting. I'd bet with a little fuel mix and a de-carb it would be OK.<br /><br /><br />Did you test it with the throttles wide open?<br /><br />-W
 
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tipitina

Petty Officer 2nd Class
Joined
Apr 15, 2004
Messages
133
Re: Compression?

Thanks Clams, you're the one I was waiting for....lol....No, I tested it in neutral (didn't think the flywheel will turn in forward)....I'm going to retest tonight, the gauge was new, but it needs to be in gear I guess? Thanks
 
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Moody Blue

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

No, I tested it in neutral (didn't think the flywheel will turn in forward)....I'm going to retest tonight, the gauge was new, but it needs to be in gear I guess?
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In or Out of gear has nothing to do with compression testing. Disable ignition (kill switch), remove all spark plugs, open throttle wide open. Connect compression gauge and crank engine at least 3-4 revolutions. Read gauge. If numbers remain on the low side, add a few drops of oil in each cylinder then try again.
 
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Motor Boater Bill

Chief Petty Officer
Joined
Jan 29, 2005
Messages
488
Re: Compression?

Did you ever figure out what the problem was with your motor? Just curious. Was the timing off? Did you get it running?
 
B

boltbreaker

Petty Officer 3rd Class
Joined
Aug 31, 2004
Messages
76
Re: Compression?

Would shaving a little off the head or installing a low compression head gasket cure this?>
 
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tipitina

Petty Officer 2nd Class
Joined
Apr 15, 2004
Messages
133
Re: Compression?

Never found out what was wrong. The wife started hounding me about the baby coming and said I have no time for a boat. (can't argue there, my first one). Compression actually came back between 110 and 118 on all 6 when I retested. Mechanic said timing looked to be off a degree or two, but that shouldn't be the problem. He said the LU, powerhead, and timing were okay, and it was likely a carb prob (like you said) or the weak transom may even be causing it to bog down to hard. I think that compression is okay, what do you think?<br />Thanks for all of your support Bill, hope to see you in a couple of years when the baby is ready for fishing and I can get another boat!!
 
Clams Canino

Clams Canino

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

Originally posted by boltbreaker:<br /> Would shaving a little off the head or installing a low compression head gasket cure this?>
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The inlines have no "head" it's a blind bore block.<br /><br />-W
 
Clams Canino

Clams Canino

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

Originally posted by tipitina:<br /> The wife started hounding me about the baby coming and said I have no time for a boat. (can't argue there, my first one
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I think you can argue (but within reason). <br /><br />If you allow your wife to put your children 1st before your own selves, it's a recipe for disaster. "Mommying" isn't parenting, thats why the 1st thing the DA tells you in Basic Training is that "Your mommy isn't here"<br /><br />Your child will spend a lifetime adapting to the world as it exists. It might as well *start* by adapting to the little world your wife and you created as you saw fit - not the other way around.<br /><br />-W
 
achris

achris

More fish than mountain goat
Joined
May 19, 2004
Messages
27,468
Re: Compression?

Originally posted by Clams Canino:<br />
Originally posted by tipitina:<br /> The wife started hounding me about the baby coming and said I have no time for a boat. (can't argue there, my first one
Click to expand...
I think you can argue (but within reason). <br /><br />If you allow your wife to put your children 1st before your own selves, it's a recipe for disaster. "Mommying" isn't parenting, thats why the 1st thing the DA tells you in Basic Training is that "Your mommy isn't here"<br /><br />Your child will spend a lifetime adapting to the world as it exists. It might as well *start* by adapting to the little world your wife and you created as you saw fit - not the other way around.<br /><br />-W
Click to expand...
I'm glad I'm not the only one who thinks this way!<br /><br />Chris.............
 
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Motor Boater Bill

Chief Petty Officer
Joined
Jan 29, 2005
Messages
488
Re: Compression?

IMHO, the most important ingredient in parenting is the two parents getting along, which requires a lot of give and take. My recollection (and it was a quarter century ago for me) is that when your wife is pregnant, it involves more give than take!<br /><br />Your only hope may be to paint the boat right away, so it looks pretty sitting in the yard, then worry about the mechanical aspects later!<br /><br />Or, sell it and get a canoe. Canoes, for some reason, are romantic. And I've done a lot of fishing from a canoe. Then work your way back up...<br /><br />Good luck!
 
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tipitina

Petty Officer 2nd Class
Joined
Apr 15, 2004
Messages
133
Re: Compression?

LOL at all of you. I already have a flat bottom boat, so that will get me through. The difficulty right now is money. I can get a little money for the boat now, and if I wait to fix it, it will just cost more later, and if I fix it now, well I can't afford it. So that is the dilema. Not the fact that we are going to be "Babying" the baby, just that I want to be "Feeding" the baby, and not worryin about a new floor and transom and motor for my old boat. And in 1-2 years when I finish my job training I will be making about 10-12 k more and will be able to upgrade anywho......my 2 cents....<br />p.s. Compression came back between 111 and 119 on all six. The first time I tested I was using the pull start rope. This time I used the key. You guys are great, and the motor on my flat bottom is an old Mercury, so be ready! The maintenance on a 5hp a little easier than a 150!!!!
 
Clams Canino

Clams Canino

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

If you use a can of Mercury Power Tune as directed your compression will come back more. <br /><br />Also adding the gas/oil mix (actually running the beast) will help too. I think it's *fine*, just needs to get treated and used well.<br /><br />I'd re-do the Ebay ad and skip the compression numbers, just say it tests normally.<br />-W
 
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