Compression question

[Gary H NC]

I did a compression check for a guy on a 1969 Mercury 80hp.
This boat and motor was imaculate.
It had 3 cylinders at 150 psi and cylinder #3 was 15 to 20 psi.

The guy had just bought the boat and it ran great on the first outing.
He told the seller he was new to boating and asked as many questions as he could from the seller.

Well the idiot seller told him to use regular gas and no mention of mixing oil.

Now of course the damage is done,

I brought cylinder #3 up on compression stroke and pressurized the cylinder.
It had a slight blow by out of the #2 cylinder hole.
I did the oil in the cylinder test and compression come up to 30 psi.

Is there any chance it got hot from lack of lubrication and lost the head gasket on just the #3?
I want to pull the head and look but is it a hopeless cause?

Very nice customer and i feel bad the numbnut that sold the boat did not even take the time to explain anything to the buyer.

The poor guy got one good day out of his new boat and that really sucks.

This motor starts so easy and i know it was a great motor before this.

Also is there a trick to drop the lower engine cowl down to access the bottom head bolts?

i will pull the head at no charge to the guy just to see the damage if i can get to the lower bolts fairly easy.
It idles good with just the miss on #3.

Thanks for any input from the Merc pros..

any tips or input would be appreciated.

 

[Faztbullet]

Re: Compression question

This engine has no head gasket ,what you see is a water jacket cover. If engine ran on straight gas all cylinders would be showing signs of damage. Since compression is 150 that is the high dome engine and the trigger is bad. Usually a Inline if #1 or #3 goes it a trigger problem, Weak impeller usually get #1,#2.and #3 sometimes

 

[Gary H NC]

Re: Compression question

Explain the trigger Fazt.....Is this motor salvageable?

 

[Gary H NC]

Re: Compression question

Bumpity........

 

[bpavey]

Re: Compression question

Gary,
Your awesome! I want to thank you again for all of your help!

 

[bpavey]

Re: Compression question

Here is something that I just noticed.
I brought the #3 piston to the top, took a screwdriver and was able to wiggle the piston ever so slightly (I'd say about the thickness of a penny).
However I was unable to this with the #2 piston.

This may be a dumb question, but should I be able to this (perhaps some wiggle for expansion)?

 

[Gary H NC]

Re: Compression question

Here is something that I just noticed.
I brought the #3 piston to the top, took a screwdriver and was able to wiggle the piston ever so slightly (I'd say about the thickness of a penny).
However I was unable to this with the #2 piston.

This may be a dumb question, but should I be able to this (perhaps some wiggle for expansion)?

Thanks Brian,
It should not have that much play in it...
I think that cylinder has lost the rings in it..

These Merc guys here are great...Fazt should comment on this when he gets back on the forum here...

 

[Gary H NC]

Re: Compression question

One more bump.......

is this 1969 80hp worth a rebuild or are parts hard to find.....?
Need some Mercury Guru help here guys...

 

[bpavey]

Re: Compression question

This engine has no head gasket ,what you see is a water jacket cover. If engine ran on straight gas all cylinders would be showing signs of damage. Since compression is 150 that is the high dome engine and the trigger is bad. Usually a Inline if #1 or #3 goes it a trigger problem, Weak impeller usually get #1,#2.and #3 sometimes

FaztBullet, Question...
When #3 chamber was pressurized (with compressor), we were able to turn the flywheel the whole time (unlike the other cylinders) with very little (if any) resistance at all. Now since there was no spark involved, the trigger would not have anything to do with this right? Unfortunatly, I did end up running it on per gas...

 

[Moody Blue]

Re: Compression question

I have a similar motor that I bought as a parts motor because "it idled bad".

Compression tests showed 155, 160, 155, 10. I tore the engine apart and found that the #3 piston was missing 1/3 of the top edge of the piston and a section of rings 2" long were gone. The ring pieces were embedded in the top of the cyl head. The walls were scored but not badly. The other three cyls looked flawless. Still debating whether or not to rebuild or keep as a parts motor.

On your motor I would pull the intake transfer port covers and take a look inside and see what you can see. With a strong light you should be able to see quite well as you slowly turn the engine over by hand (remove all spark plugs first and disconnect battery). Hopefully you can get away with just replacing #3 piston and rings and reboring cyl #3.

 

[bpavey]

Compression question - Diagram

Compression question - Diagram

I have a similar motor that I bought as a parts motor because "it idled bad".

Compression tests showed 155, 160, 155, 10. I tore the engine apart and found that the #3 piston was missing 1/3 of the top edge of the piston and a section of rings 2" long were gone. The ring pieces were embedded in the top of the cyl head. The walls were scored but not badly. The other three cyls looked flawless. Still debating whether or not to rebuild or keep as a parts motor.

On your motor I would pull the intake transfer port covers and take a look inside and see what you can see. With a strong light you should be able to see quite well as you slowly turn the engine over by hand (remove all spark plugs first and disconnect battery). Hopefully you can get away with just replacing #3 piston and rings and reboring cyl #3.

Thanks for your feedback Moody Blue!
Based on your suggestion and referencing the accompanying diagram where in the heck would the cover actually be located. The diagram, atleast to me, is vague. Thanks in advance!

 

[Gary H NC]

Re: Compression question

I think it would be the exhaust plates in the diagram.
We could pull it and take a look..
What do you think?