MerCruiser 120 Compression low in one cylinder

[RMELAN]

I have a 1968 MerCruiser 120 2.5 in my restoration Glasspar. I just did a compression check, all pugs out, throttle open wide. I have 3 cylinders with 120lbs and one cylinder with 100lbs. I have not started the engine yet. It needs points, plugs, etc. How does this impact how the engine runs? Would it a a valve possibly?

 

[todhunter]

It's far enough off to where you should correct the issue. You'll need to do a leakdown test to determine what is causing the low reading on the fourth cylinder. Valves are one of several reasons for a low compression number.

 

[RMELAN]

The engine has been sitting for years I believe. I heard if you use a little Marvel Mystery Oil in the top of the cylinder and let it soak it may loosen some of the carbon deposits? Any experience with this method?

 

[Alumarine]

You should run it for a bit and check the compression when warm before trying to figure out what it may or may not need.

 

[RMELAN]

Thanks, do you know what the compression should be when warm for the Merc 120?

 

[Bondo]

Thanks, do you know what the compression should be when warm for the Merc 120?

Ayuh,...... Welcome Aboard,........ 'bout 150 psi is a newish healthy motor,.......

100 psi is a rebuildable core motor,......