1989 merc 115hp compression

[Prentissboater]

Im looking at an 89 mercury 115hp and was wondering about compression. I performed a test on it, cold engine and all cylinders checked dead on 110psi. Is this ok? Thanks

 

[GA_Boater]

Welcome aboard.

Having even numbers is a good sign. Has the motor been sitting and not used for a while? Probably most important - How does it run?

 

[QBhoy]

Wee tiny bit low, but Acceptable I'd say.
Few variables as you know. If she isn't warm with rings expanded, will always read a bit low.
Presumably throttle was open too.
Unless the gauge is calibrated, can be a bit of a guess too, but if they all the same roughly and above 100, I'd say nothing to worry about.
Good luck.

 

[DeepCMark58A]

Good to see that the readings from cylinder to cylinder are close as well.

 

[Prentissboater]

Motor was cold had only been ran about 10 seconds before i done the test. I forgot to hold the carbs at wot, they were just at idle. Supposedly it hasnt been ran since october and it fired right up. Sounded good. Thanks everyone.

 

[Prentissboater]

So will compression get better with a warm engine and wot or worse? What should my compression have beeen? The guy is asking 1200 for motor and controls, has a stainless prop, is it worth it you think? Its a clean motor.

 

[racerone]

Warm or cold does nor matter.----Wide open throttle does not affect compression either !

 

[QBhoy]

Traditionally, compression will see better results when the engine is warm......of course.

 

[QBhoy]

The idea behind wot, is to give the engine the best chance at breathing in as much air as possible when turning over during a compressions test. i am not certain why raceron has said this.

 

[racerone]

Because ports are wide open before compression starts !!

 

[QBhoy]

Was more referring to the warm engine bit racerone. Take your point on the 2 stroke engine, but standard practice, I think you'll find.

 

[GA_Boater]

Why do motors start at idle, both 2 and 4 stroke. Has anyone checked compression at idle and wot and seen a repeatable difference?

 

[Faztbullet]

Has anyone checked compression at idle and wot and seen a repeatable difference?

On Merc and Johnrude V-6 motors with idle relief notch/hole there is a 10-15 psi increase from idle to WOT per the factory!!!

 

[Prentissboater]

Thanks guys.

 

[Prentissboater]

I went on and bought the motor, does anyone know if these motors are reliable.

 

[racerone]

At idle they only run on the top 2 cylinders.---At higher ( above 1800 rpm ) the bottom 2 carburetors kick in.--In my opinion they are not meant for prolonged low speed operation.

 

[QBhoy]

if it has oil injection, disconnect it and mix the tank 50:1....and it will be. Like previous poster mentioned. Might foul the plugs at sustained low revs.

 

[QBhoy]

Is it an inline 6 cylinder or 4 ? '89 must be close to the change over period.

 

[Prentissboater]

Its an inline 4

 

[gbrowne9]

I have had a 115 4 cylinder for 8 years and it been one of the best engines i,ve owned.great on gas.