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#10892321 - 06/03/15 10:46 AM Compression on a Mercury
Fisher77 Offline
Extreme Angler

Registered: 07/11/10
Posts: 1119
Loc: Lubbock, TX
I am looking at a boat with a 1986 Mercury 175 Black Max on it. Before I buy, I want to make sure the engine internals are sound. Can anyone tell me what the compression should be on this engine? What psi is ideal, and what psi would indicate needing to rebuild it. Any help is appreciated.

Thanks
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#10892545 - 06/03/15 12:29 PM Re: Compression on a Mercury [Re: Fisher77]
gary purdy Offline
TFF Guru

Registered: 05/24/03
Posts: 10508
Loc: Littlefield Tx & Lake A. H. US...
A motor 30 years old is a roll of the dice but all 6 at 100lbs compression is good, under 90 I would be concerned. +or- 10% is the general rule
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#10892547 - 06/03/15 12:30 PM Re: Compression on a Mercury [Re: Fisher77]
Stump jumper Offline
TFF Celebrity

Registered: 04/19/04
Posts: 6983
Loc: Rockwall
I have the same motor in a 1985. I do not remember where mine was before I had the heads milled. I am thinking 120s. Right now it is 140. Make sure all cylinder are with a few lbs of each other.

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#10892786 - 06/03/15 02:12 PM Re: Compression on a Mercury [Re: Fisher77]
TLW Online   content
Extreme Angler

Registered: 11/16/06
Posts: 2004
Loc: ft worth
if ya can get hold of another gauge it would help... need something ta compare it to unless that one is a KNOWN good gauge.....100 lbs is a little weak at any age for 'em.. need 115 minimum.. has tha motor been open at anytime.. in other words does it still have all 6 "chrome" cylinders?...

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#10893248 - 06/03/15 05:10 PM Re: Compression on a Mercury [Re: Fisher77]
Fisher77 Offline
Extreme Angler

Registered: 07/11/10
Posts: 1119
Loc: Lubbock, TX
Thanks for the replies.

Do you test dead cold, or at operating temp?
_________________________
“You may all go to hell and I will go to Texas.”....Davy Crockett

"Victory or Death"....William Barrett Travis



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#10893341 - 06/03/15 05:47 PM Re: Compression on a Mercury [Re: Fisher77]
Pat Goff Online   content
TFF Team Angler

Registered: 01/29/05
Posts: 4985
Loc: Marble Falls/Seadrift
Warmed up is better, but dead cold will show a cancer case.
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Marble Falls/Seadrift TX

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#10894328 - 06/04/15 07:18 AM Re: Compression on a Mercury [Re: Fisher77]
gary purdy Offline
TFF Guru

Registered: 05/24/03
Posts: 10508
Loc: Littlefield Tx & Lake A. H. US...
Reason for my compression #`s. I found most will not warm up the motor or check at full throttle. I personally do not do a cranking compression test any longer. Leak down and running compression tells me so much more.
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“The world will not be destroyed by those who do evil, but by those who watch them without doing anything” GOD IS GOOD

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#10899157 - 06/06/15 09:01 AM Re: Compression on a Mercury [Re: Fisher77]
Fisher77 Offline
Extreme Angler

Registered: 07/11/10
Posts: 1119
Loc: Lubbock, TX
Thanks for the help everyone.
_________________________
“You may all go to hell and I will go to Texas.”....Davy Crockett

"Victory or Death"....William Barrett Travis



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#10905675 - 06/09/15 12:30 PM Re: Compression on a Mercury [Re: Fisher77]
Muzzlebrake Offline
Extreme Angler

Registered: 12/26/14
Posts: 1004
Loc: Euless
When I was shopping for a used boat I took my compression gauge with me to test the boat's motor. Anybody objected to testing the motor the conversation ended right there and on to the next one. I would be looking for 115-120 each hole and not over 5 psi difference between any hole. 100-110 is iffy and below 100 is a no go for me. Also a motor can have good compression and still have poor leak down. Leak down is a bit harder to check and needs a special gauge and you have to know how to put each piston at TDC
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#10906769 - 06/09/15 07:56 PM Re: Compression on a Mercury [Re: Fisher77]
leanin post Online   content
TFF Celebrity

Registered: 08/18/09
Posts: 5381
Loc: TEXAS
sometimes carbon build up can affect one or nmore cylinders I had a motor go from 85 to 110 after a decarb.. one cylinder was low. If them rings aint spinning they aint sealing.
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#10922792 - 06/17/15 11:06 AM Re: Compression on a Mercury [Re: Fisher77]
DAN-O Offline
Extreme Angler

Registered: 02/21/04
Posts: 2441
Loc: MANSFIELD, TEXAS, USA
Mine is a 1997 Mercury 150 EFI. It has compression of 125-130 in each cylinder.

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