Mercruiser 120 compression questions

[Chris the Prop]

Hey All,

I have a 76 Mercruiser 120 2.5 GM. It cranks great and runs fine until you place a load on it, then the RPM's drop and seems like the engine just bogs down. I had a compression test and all 4 cylinders were reading 80-90. Oil in the cylunders brought the compression back up to about 120.

My question is this. I have 2 options . rebuild or replace. I gave found a replacement engine rebuilt in 2007 that has compression 110-110-115-115 for about the same price as the rebuild parts alone.

What is the normal compression range for this motor? I've been told everything from 100 to 180 in each cylinder. I'm thinking of buying the replacement engine, and then rebuilding the original one anyway to drop in in the future, but I don't want to buy another engine that needs rebuilding right away.

Thanks for the help!!

 

[6meter]

Re: Mercruiser 120 compression questions

If I remember correctly a recent thread stated that their compression was at 160 for their 4 cyl.

 

[Fordiesel69]

Re: Mercruiser 120 compression questions

You did your test wrong.

-Battery charger set to 50 amps or more.
-Throttle open wide, with choke also open.
-All plugs removed.

You shoudl be getting in the 140's if the engine is good. 120's is getting tired, and less than that and you have a glorified trolling motor. A wet test should have brought you up more if the rings were worn. Maybe a valve job just needs done.

 

[atomb]

Re: Mercruiser 120 compression questions

that was mine it was 160 with brand new rings -- should be 140- 160 is perfect compression i believe. please correct me if im wrong and my compression test was cold so it might not be as acurate. and it was a 140 3.0 not a 120 . dont know if that matters the manual should tell youy what the compression should be.

 

[Bondo]

Re: Mercruiser 120 compression questions

What is the normal compression range for this motor?

Ayuh,... I'd call a motor at 140/ 160 a New motor....
I call a motor at 100 a Rebuildable Core.....

Properly completed Tests of course....

 

[woosterken]

Re: Mercruiser 120 compression questions

the pocket service guide , that came with my '74 120hp says compression 140 psi with a 20 psi plus or minus between cyclinders.

woosterken