What should compression read?

[swick]

I've got a 99 Merc force 120 horse 4 cylinder 2 stroke. What should the normal compression be? It hasn't been run in about 2 years and I've not taken it out in the water yet. Current readings are 110, 180, 180, 160. (I ran engine for about 15 minutes in driveway before compression test).

 

[Silvertip]

Re: What should compression read?

Most manufacturers don't even publish compression test numbers anymore but if they are over 100 PSI and within 10% of each other the engine will run ok. In your case #1 is down significantly from the others indicating something is amiss. However, I suspect a good decarb (decarboning) with SeaFoam might bring that number up closer to the others. Use the "search" function in this forum to find the "decarb" process.

 

[BLU LUNCH]

Re: What should compression read?

180 PSI ? I never saw a reading that high. My V-6 Mercury was 140 after a fresh rebuild...........

 

[Boss Hawg]

Re: What should compression read?

Most manufacturers don't even publish compression test numbers anymore but if they are over 100 PSI and within 10% of each other the engine will run ok. In your case #1 is down significantly from the others indicating something is amiss. However, I suspect a good decarb (decarboning) with SeaFoam might bring that number up closer to the others. Use the "search" function in this forum to find the "decarb" process.

Agree-
Worked for mine

 

[swick]

Re: What should compression read?

I put a bottle of seafoam in the tank not too long ago, didn't make a notable difference, I've also been told to just pour some straight into the cylinders (haven't done it though), any thoughts on that?

 

[jeff_smith_0423]

Re: What should compression read?

Running it through the tank might free up some varnish in your carbs and fuel system, but what is being recommended is a full on "shock treatment" decarb. I'm sure the directions are around here someplace.