1979 MerCruiser 260 compression

Hi guys,

I'm considering buying an old 1979 Carver Montego with a 5.7L Mercruiser 260 V8. I had it inspected by a local shop and they ran a compression test on all the cylinders with results that concern me.

Now he didn't run the engine to full operating temp, and this was the first time it has been run since it was winterized. The current owner blamed the low compression on sitting for awhile and the fogging oil used for winterization.

Does these compression numbers look right to you?



The engine supposedly only has ~70 hours on it, the current owner replaced it after buying it at auction with a cracked block.

Welcome aboard !

The new motor installed had a cracked block ?

These numbers are not good, anything below 100 is not good. The low numbers could be the valves, or a head gasket or rings.

If you are asking should you buy this boat, i have to say pass on it.

My 86' has numbers close to factory new specs and i did nothing to it.

Thanks for the reply, this is what was afraid of.

The current owner replaced the original cracked engine with a new one four years ago. The engine supposedly has only 60 hours on it. He insists that it has low compression readings because they were taken with the engine cold after sitting all winter. He's telling me that if the engine gets run for awhile compression should come up. Is there any truth to this?

What are the factory specs for compression on this kind of motor?

Now he didn't run the engine to full operating temp, and this was the first time it has been run since it was winterized. The current owner blamed the low compression on sitting for awhile and the fogging oil used for winterization.

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Incorrect! He's apparently more concerned about selling, than being honest.... or he himself is being misinformed.

In addition, the #5 cylinder reading is too far below the highest cylinder reading, according to many recommendations.
In fact, and I'll agree with Chief here, all pressure reading are too low for this engine.

Now..... here's a thought:
If the short block assembly ONLY has been replaced (as he suggests was 4 years ago)....., it's very likely that the wrong pistons for the existing cylinder heads were used.
(trust me..... I've seen this many times!)

IOW, if the previous 5.7L SBC engine had Flat Top pistons installed (very likely in 1979), it would have been properly equipped with the 76cc cylinder head chambers, in order to render a correct SBC Marine Engine C/R.

If these same 76cc cylinder heads were re-installed on a fresh short block that was equipped with the dreaded "full dished" pistons (which are very common today), this would likely explain the over-all low cylinder pressure readings.
Not a good combination!

We CAN NOT arbitrarily change cylinder heads with incompatible pistons..... and visa-versa!

I'd get hold of a Bore Scope, and take a peek into a cylinder.
Today's bore scopes are pretty amazing, in that you may be able to see the piston deck, and determine which one it is!
I.E., Full Dished, or Flat Top!

The cylinder head casting numbers will tell you the combustion chamber volume.
If this was an over-the-counter GM short block, the block casting numbers may tell you which pistons have been used.
If re'man from local shop...... it's a crap shoot!

An unsolicited note:
If the boat is what you are looking for, and if priced right, replacing an engine is one of the lessor evils of all the potentials that one could have......, IMO.
Price and your willingness to do this, would be a Key Factor!

Good luck with it.

.

He insists that it has low compression readings because they were taken with the engine cold after sitting all winter.

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Ayuh,... He's trying to Sell his boat...

I'd look for a Different 1....

Those compression #s Suck,.... Walk away....

I'd have to suggest that more info is needed.

Like I suggested earlier....., and I'll elaborate some;
Given a choice between a poor condition hull, poor bright-work, poor fittings, poor Teak Wood, wiring and hoses issues, or you name it... Yet with a good engine -vs- a boat where all of the above are in great condition, yet needs an engine ......... an engine replacement is often a far cry lesser issue than some of the for mentioned. IMO, that is!

That's why I suggested:

If the boat is what you are looking for, and if priced right, replacing an engine is one of the lessor evils of all the potentials that one could have......, IMO. Price and your willingness to do this, would be a Key Factor!

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An engine can be replaced in one day, and we gain the benefits of fresh power!

If my first hunch were to be correct, a simple cylinder head replacement (correct chamber volume) may cure this.... don't know... more info!

Who knows.... this boat may have been under-powered since this new short block was installed, if the heads are wrong.

Just saying that eveything needs to be considered first!
IOW, don't walk away from a great hull, and one that you love, over an engine issue alone!
There are much worse issues that one could come up against.

.

Lots of good info here, I appreciate all the replies!

The boat isn't quite everything I was looking for, as it needs some work in some other areas.

http://s195.photobucket.com/albums/z162/tomboy376/1979%20Carver%202357/

He's selling it for $2,900, and I was interested in it because I figured this would be something that I wouldn't have to put a lot of work into it. I took him at his word when he mentioned the new engine, and the rest of the boat is structurally sound. So I figured this would be a cost-effective way to get my family out on the water without worrying about an engine or structural failure.

Rick, great info about the possible wrong cylinder heads. Makes sense to me, but if the wrong heads were used for the pistons it would still not explain why there is 90 PSI in one cylinder and as high as 125 in others.

I'm capable of minor engine work, and I'm willing to learn more major stuff. But as it sits I'm going to have to invest more than $1,000 in the boat and trailer excluding any potential engine problems. The mechanic told me the bellows would need to be replaced in the next year or so, and he quoted me $850 to do those.

Many issues to look at here for that boat.

1, 1979 if it has a fiberglass gas tank that is a bad thing with the 10% alcohol in the gas.
2. No wonder the motor may have low compression, That boat has a mouse motor in a BARGE!!! WAY WAY under powered.
3. That motor should have ~ 150-170 psi per cyclinder. I would think.

I agree ~ $5000.00 in repairs to make it a worry free boat.....

If you could afford that then I may be worth it as it is a CARVER and it is a large boat!


Walk away..........

Yeah, much to think about.
At 23 foot, you're still under 26, so the A drive should hold up OK.
And you are well within the HP limits of the A drive.

Several years ago, I converted this same boat/same engine (customer's boat) to a V/P Duo Prop.
With the standard 5.7L, it changed the characteristics of the boat considerably.

This may have bee too big a boat to start off with to begin with.

Too bad your not near me, i am just tired of my boat that is in perfect condition mech. Wise, with a good trailer. 140 hp motor cheap on gas. I has new shafts, new coupler , and perfect compression.

Anything that would need to be replaced has been. your good to go for at least 5 years.

I am just tired of the boat. I want a walk around with 2 motors to go out to the canyons for the really big fish. For me i need at least a 25' in short i am ready to move up.

Maybe you should look for a 19' 3.0 will sit 6. Maybe something with a outboard.

Just depends on what your looking for in speed and comfort. Gas will be close to 5 bucks a gallon mine runs all day 8-10 gallons. That big one your want would have been double at least.

Good luck, i wish you well in your endeavor to purchase a boat for your family's enjoyment.

My family and i always seem to bring up the adventures we had on our 3 boats.

I also have a open bow 16 ' with a 70 hp outboard.

KG brings up an excellent point re; the fiberglass fuel tank and E10 fuel. You'll want to verify this.

I just looked at the pictures again while trying to see if the value is there (boat and trailer).
Is that a Caulkins trailer?
The one using a round tubing frame-work roller/support system?

If so, have these tubes looked at thoroughly.
Caulkins earned a reputation regarding these tube frames.
Tap on each one checking for the dreaded "thud" sound of rusted out tubing interior.
If they "ring", then they may be OK.
If NOT.... you will have trouble with them.

Chief, not sure how/why you think that a 23 foot boat is too large! I consider a 23 to be in the smaller boat category..... and certianly not out of the range for that particular engine/drive package when in good condition and when running correctly.

.

Just want to pile regarding the compression numbers.

Just did a compression test on my engine (5.7L SBC), following some extensive repairs that did not include any work to the cylinders, pistons or rings.
I did the compression test cold, after the engine sat for 4 months.
The lowest I had was 150psi.
And this engine has some hard hours on it.
WRT the fogging oil... this would tend to help, not hurt the compression.

Hope this is a good comparison.
Unless you want to put an engine in it (or do a crap load of work), don't just walk away, RUN!

One more thought... for at least your first boat buying experience, hire a surveyor. It'll be the best $300-400 you'll ever spend.
Seems like a lot but it's well worth it.

troym said:

WRT the fogging oil... this would tend to help, not hurt the compression.

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Normally yes, with regard to piston rings......, and is why I said; "Incorrect! He's apparently more concerned about selling, than being honest.... or he himself is being misinformed."

None of which necessarily means to RUN from this boat..... just use it as a bargaining tool re; price..... if the boats suits the needs.

Why i thought it was a 26 i don't know. 23 is nice.

I feel a older boat is perfect to learn on. 19' -21' maybe 23' but then that long, to learn to operate safely.

I'm looking 26-30 going 5-7 out and 5-7 hours back. And staying in the Canyons for the weekend. 4 - 5 people.

Thanks everyone for your replies. This would have been the first boat I would own, but I spent a lot of time on boats growing up. The size didn't concern me much, but mechanical reliability certainly did.

After seeing what you guys had to say, as well as some others I contacted on this, I decided to walk away from this boat. It was borderline as it was considering how much I would have to put into it and the trailer. I was seriously considering it because of the supposed new engine installed just a few years ago. NADA says the boat is worth $2,900. Buying it for that, spending $1K on other sundry items, and then possibly putting a new engine into it seems foolish at best.

Stinks to spend all the time and money I did checking this thing out, but it would be much worse to replace an engine on a boat that's 7 years older than I am. I'm gonna have to keep looking, but after learning what I have I'll be looking for something with an outboard.

Once again, THANKS!