Re: Compression Check Results
Lower compression is caused by a few things.
Simplest things first...check the plugs for indications, your generic manual should tell you what on earth is happening to your plugs, and also see if they are all of the same heat and brand name, you wouldn't believe what difference that can make over a short time!
And...., did you have your gage hooked up properly for those two cylinders?
Ring wear, could be due to hotter running then the rest, and because both cylinders #6 and #8 are "aft" on the engine, it could be anything from bad coolant circulation, to partially obstructed water jacket areas from settlement of scuzz in the coolant, not likely, to just rust from sitting, cross leaking from a bad set of head gaskets, also not likely, or just two sets of rings being from a different lot. Ditto for the valves.
Valve and guide wear, check for smoking at startup or when the engine is gunned (constant smoking is usually ring related.)
Do a dry check first then a wet check to determine whether it is valves or rings.
Guide wear could be from anything lubrication related to rocker problems, cooling, different valves for those two cylinders, or hot running for any of the above reasons.
Check PCV valve if so equipped, if it is plugged, you could have excessive blowby that could point to ring wear.
You also have to wonder who has been in the engine in the past and what parts or methods were used to do any work in those areas if any.
As for a guess as to why it has that low compression, pick any one of the above, any one's guess is as good as another's, but if that problem is already going on in there, it needs to be fixed without a doubt! Add-to fluids or mechanics in a bottle are never a good thing even if it "fixes" the problem, the problem arrived for a reason!...Your own and other's safety may be at stake.
The compression for those two cylinders is low but not totally unacceptable in Bubba's land lubber vehicles, on water it is a different story, and your life may depend on your better judgement or where you are going to do your helpless floating around if that happens.
For me....I wouldn't want that situation in a boat at the best of times (yep, learned the hard way.)
My own standards dictate that these pressure differences are too big in an engine that is constantly trying to hammer itself into pieces, especially on a boat.
Differential pressures cause wear and tear and breakdowns at the most inconvenient times guaranteed!
I have been stuck out in an ocean with a bad cylinder before. I fixed it on the go because I am a fanatic about spares for those engines that are workable when you are out there, and I'd done it before on land.
Try hand lapping a valve in on a 75 year old engine while bobbing around looking like dinner to the local inhabitants, tied to an anchor bucket, exciting at the least.
I shouldn't have been out that far in the first place with the old girl, but you learn and hopefully live to respect that.
But I'm sure you know all of that already
My advice?....don't even think about it, fix it it'll be cheaper that way.
PH.
I know I'm long winded, that's from blowing on the coals to get some action out of the boilers!
