LM318 Compression Check (and other mystery things)

[Mac32]

Ok I changed the plugs in my port engine engine today (starboard still to be done) and had a few small things that made me scratch my head when I did the compression check. I will start from step one for all you gearheads that may catch something in the details.

1) Warmed the engine up to 145 degrees

2) Pulled all the plugs out and noticed wet fuel around all the threads and all 8 were heavily carbon deposited (yes rich mixture)

3) Also pulled the spark arrestor off to open the choke and throttle plates for the compression check but noticed wet oily deposit around the ring of the carb where the spark arrestor sits on it. This one is strange. Why would top of carb area be wet it looks like fuel but feels and smells like oil. (this is on the Starboard engine also)

4) Preformed a compression check on all cylinders and all 8 were between 80-90 lbs Last time I did a check in 1999 they were all between 110-120 (remember the boat sat for 5 years in between so it has only had 4 years of use).



THE GOOD -
The engines run beautiful, and smooth (but rich), so smooth I can set a glass of wine on the carburetors during idle. They start every time with instantly and oil pressure during idle after running HARD is 45-50 and 70 at 2500 rpm. They also magically stopped leaking oil, not a drop this season.

THE BAD
Low compression, could this be causing it to run rich or is running rich washing out the cylinders and causing low compression. The rich fuel exhaust smell is almost as unbearable as a driving behind a school bus (but you cant see it).

Also I can hear the lower end is a little loose there is a deep knock in both engines very quiet but its there. I call it rod knock that you can only hear when you get down low in the bilge.

The engines have been rebuilt before I got the boat and I think (not sure) they were rebuilt at 1000 or 1200 hrs, and they now have 1800 on them.

Now here is the question, I am planning on a rebuild but not for another season or two what else can I do to check the health of the engine or get compression up.

SO WHAT DO YOU THINK?????

[prowlersfish]

I will give you my opinon that come with 30 plus years of working on cars trucks boats and most anyting with a engine for a living .

If a engine running good there no real need to do a compression test. the only thing it will do is worry you .If runs good run it , yes running rich could wash down the clnyders and lower the compression. I would find out if its running for sure and take care of that ,and for get the compression test . everthing I own runs good .I don;t know what the compression is and don't care.

Engine knock , you will hear things in a boat that you would never hear in a car or truck . you can hear knocks when there is nothing wrong (sound with reflect from the boat hull. ) you should hear a diesel in a steel or alum. hull, Just keep a ear open , compair it to other boats . without hearing it my self its hard to say for sure

as far as the starboard engine what would you do if you get the same readings ? worry more or less ? I would forget the compression test

I am sure some/or a lot will disagree . but I got 30 plus years a pro to back up how I feel.talking to techs overseas (diesel techs)they think we are obsesed with compression test .I think there right

[Mac32]

Thanks Paul!!,
Thats what I have gathered from most mechanic friends I spoke with, but the only thing baffling is the oil ring I get around the top of the carburator under the spark arrestor......On both engines. I will try to get a pic

[prowlersfish]

Post a photo , but that is not abnormal to get some oil there under the load a marine engine is under you will get some oil vapors and this were it ends up at. this from blow-by and all engine have some I see gummed up throttle plates on cars with 15000 miles on them (from oil)