1995 Mercury 115 HP OB what is good compression?

[thebrain]

1995 Mercury 115 HP OB what is good compression?


I’m concidering buying a older boat it has a Merc.115HP OB
The seller has stated that the engine was serviced in 2011
I’m unsure of what was done?

Engine please provide tips to check it’s condition like

1.what is good compression?
2.how will I know if the waterpumps impeller is still good?
3.should some lower units oil be drained and inspescted?
Thanks TB

 

[thebrain]

1995merc.

 

[thebrain]

I'm thinking 120PSI is desireable?

and each cylinder be within 10PSI of each other?

 

[Bayou Dave]

I'm thinking 120PSI is desireable?

and each cylinder be within 10PSI of each other?

Within 10% of each other, which may or may not be 10 psi of each other.

 

[thebrain]

so what is the desireable PSI? Thanks

 

[Bayou Dave]

Around 110 to 140 ish

 

[flyingscott]

Several things you need to know about that motor. I would not buy without a test drive or a warranty. The motor is a 2+2 that means it idles on the top 2 cyls and the other 2 cyls kick in at 1800 rpm you want to make sure that is working correctly. Also not known for not having the greatest stators. That being said they are good motors that work well I had the 125 version. I liked it quite a bit and wished I still had it

 

[tommarvin]

Whenever you buy an out board, you always change the impeller, check for water in the LU gear oil, before you buy, after you buy drain and fill with 100% synthetic marine gear lube, its tested with 10% water contamination in it, and passes use the correct seals on drain and fill, put thermostat in a pan of cold water, slowly heat it up what temp does stat open ? 50-1 two stroke oil, in the gas, do you have to ground the spark plug wires to the block for a compression test ?

 

[Texasmark]

The service manual covering that engine and my last engine states that anything below 120 MAY be cause for concern. As others have stated, being similar in compression is more important in diagnosing a potential problem unless it's obvious that the engine is at it's end of live and time for rejuvenation. Obviously many things affect the reading especially with looper engines having only 2 pressure backed piston rings that have to move freely to function properly.

 

[DavidMoore]

My 2 cents worth, mostly repeating what others have said.

Most DIY compression gauges are not that accurate so don't get too hung up on the actual readings, so long as they are up there around the 120 mark and within 10 % of each other you should be good to go. The 10% rule is the most important.
Check the plugs for fouling, (or unusually clean which indicates a water leak) at the same time.

Don't buy without a test drive, knowing that the motor starts well, sounds healthy, and performs well on the water will tell you most of what you need to know.

Yes check the oil but after the test drive, (It might have been refreshed yesterday).

Check all the fuel lines are in good condition, deteriorating fuel lines can lead to all kinds of bother. If you touch them and they leave black marks on your hands consider them needing to be changed.

Last serviced in 2011, that's 4 years ago. Get a manual and give it a service yourself it's really not so difficult.

IMPORTANT --->>>>>> You don't know the impeller is in good condition, don't take the risk change it. You will then know for sure.