Compression

sr79

Seaman Apprentice
Joined
Mar 22, 2014
Messages
49
1979 Mercruiser 260 HP - I have 105 to 120 PSI compression. And the exhaust really smells bad, not oil but maybe un-burnt fuel. How low is too low for compression and does stinky exhaust mean bad valves?

Thanks,
sr79
 

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,093
Re: Compression

i woould think that 145+ would be a good engine. did you do the test correctly? Are these readings after squirting some oil into the cylinders
 

Bondo

Moderator
Staff member
Joined
Apr 17, 2002
Messages
70,465
Re: Compression

1979 Mercruiser 260 HP - I have 105 to 120 PSI compression. And the exhaust really smells bad, not oil but maybe un-burnt fuel. How low is too low for compression and does stinky exhaust mean bad valves?

Thanks,
sr79

Ayuh,.... Welcome Aboard,.... Nope,... Fuel smellin' exhaust says ya got a Carb problem,....
 

JustJason

Vice Admiral
Joined
Aug 27, 2007
Messages
5,319
Re: Compression

You need 100psi minimum across all the cylinders for it to run and idle properly. Your getting down there, and on your last legs. But it should still run.
 

84EdH

Chief Petty Officer
Joined
Apr 30, 2011
Messages
575
Re: Compression

sr79, how did you do the compression test?

engine warmed up?

throttle pushed all the way forward?

must turn engine over 4 or 5 revolutions also.

the readings, also, as stated squirt a teaspoon of engine oil down the spark plug hole and re-check the compression. if I goes up fair amount, it suggest rings are bad.

fuel smell in oil as Bondo said is probably a stuck float in the carb.
 

sr79

Seaman Apprentice
Joined
Mar 22, 2014
Messages
49
Re: Compression

sr79, how did you do the compression test?

engine warmed up?

throttle pushed all the way forward?

must turn engine over 4 or 5 revolutions also.

the readings, also, as stated squirt a teaspoon of engine oil down the spark plug hole and re-check the compression. if I goes up fair amount, it suggest rings are bad.

fuel smell in oil as Bondo said is probably a stuck float in the carb.

------------------------------------------------------------------------------------------------------------------

I did another compression test this morning with throttle wide open and engine warm.

The new readings are: 1-120, 8-111, 4-111, 3-120, 6-108, 5-120, 7-118, 2-120
Note: Each cylinder would come up to 40 PSI +/- with 5 pulses and then to recorded reading after 15-20 pulses.

Next - I squirted a little oil in cylinders #1 and #2, No Change.

The engine is hard to start after setting a few days but then starts quick, idles and sounds good, well, OK.

The carb was recently gone through but still uses LOTS of gas. Spark plugs are black but not fuzzy.

Still need suggestions for fixing stinking exhaust, reduce gas consumption and get more power. Do I need to get a valve job, a complete overhaul, new carburetor or ?

Thanks,
sr79
 
Last edited:

old islander

Petty Officer 1st Class
Joined
Jan 27, 2013
Messages
302
Re: Compression

Black plugs would indicate a overly rich fuel condition...fuzzy or not. Follow bondos advise. Go through the carb again. And also verify your choke is working properly.
 

midcarolina

Chief Petty Officer
Joined
Jul 16, 2013
Messages
631
Re: Compression

If numbers didnt come up after squirting oil in the cylinders the next step I would do is a leak down test of each cylinder.....

That will help pinpoint ifn it's a valve issue.... I think you have two seperate issues #1 carb issue #2 compression
 
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