Low compression...(60/140)...Expected scope of work?

[matt 57]

Ran compression test on '77 Merc 888 302 block:

#1 - 120 (140 after oil injection)
#2 - 60 (140 after oil injection)
#3 - 140
#4 - 140
#5 - 140
#6 - 140
#7 - 140
#8 - 120 (140 after oil injection)

I understand that it's hard to say before you see it taken apart but it would be nice to hear your thoughts guys.
Complete overhaul?
Just one cyliner?
A single cylinder with very low compresson...how it happens?
Is it normal to go from 60 all the way up to 140 after oil injection?

Is there any way to recognize whether the engine was rebuilt?

Will appreciate any input. Thanks.

 

[Bondo]

Re: Low compression...(60/140)...Expected scope of work?

Is it normal to go from 60 all the way up to 140 after oil injection?

Ayuh,... 60psi Isn't normal anyways,....
I'd say it's Rebuild time.... Looks like the rings are Shot...
I've never heard of rebuilding Just 1 cylinder....

 

[45Auto]

Re: Low compression...(60/140)...Expected scope of work?

It's a 30 year old motor. Don't nickle and dime yourself to death. Once you pull it out either rebuild it completely or replace it. If you just band-aid the one low cylinder you'll be pulling it again next summer to fix the other 2 low ones, etc, etc.

 

[matt 57]

Re: Low compression...(60/140)...Expected scope of work?

Don't nickle and dime yourself to death.

I'm not going to. I knew the engine will need work at the time I bought the boat.
Just would like to hear from experienced mechanics why something like this happens when a single cyl much lower than all others and whether it is a common occurance.
As per service manual tune-up specification 140 is norm and 120 is still in acceptable range. Will pull it out and see...

 

[Bondo]

Re: Low compression...(60/140)...Expected scope of work?

Just would like to hear from experienced mechanics why something like this happens when a single cyl much lower than all others and whether it is a common occurance.

Ayuh,.... It's 30 Years Old,+ you don't know Any of it's History....

It's Probably been Over-heated a time or 12....
Water back in the exhaust is a Ring Killer....
Hydro-lock,...
Leakin' riser gaskets,...
Rusty cylinders from Lack of care,...
Who Knows,..??

Just alittle rule of thumb I go by,...
200psi,+ Up is a Racin' motor...
175/ 200 is a Powerfully built motor.... a Real Runner...
140/ 175 is a production motor, in reasonable to Good shape....
100/ 140 is between getting 1 more lap outa a Dead Horse, to a tired production motor....
Below 100psi Is a Dead Horse.......

 

[jerryjerry05]

Re: Low compression...(60/140)...Expected scope of work?

Before you rebuild.If it's sat for a while.Try MarvelMysteryOil. Fill up the cylinders and let it set for a week.Then start it and let it come up to temp.
Do another compression test,then decide if you want to do a rebuild.J

 

[havasuboatman]

Re: Low compression...(60/140)...Expected scope of work?

JerryJerry, he's not trying to sell it.

 

[matt 57]

Re: Low compression...(60/140)...Expected scope of work?

Thanks Bond-o.
I do know this is a mass production motor and 140 was a top as per factory specs. And this was the reason I posted my question about 60/140. I used to check compresson on a couple of cars but never came across something like this as I don't do it for living.
Another thing I'd like to know yet : when compression goes up from 60 to 140 after injecting of 10 ml of oil on the top of the piston, does it mean anything specific? I'd rather expect 20-30 psi increase, but this much?

About the age...age wont kill a motor...owner and bad mechanic will...
I bought her for a little money just to gain some initial experience and not going to pump a fortune in this motor, unless I fall in love with the boat after I invest some of my labor...which is possible as I like '70s.

 

[matt 57]

Re: Low compression...(60/140)...Expected scope of work?

JerryJerry, he's not trying to sell it.

Sorry guys, your posts showed up while I was typing mine. If you read the previous one you will understand what is this about.

I pulled her out of lake myself, everything was working...just weak motor and some minor issues... Winter project.

 

[jerryjerry05]

Re: Low compression...(60/140)...Expected scope of work?

Did I miss something??? I said nothing about selling!!!
I gave a possible fix.I have done that to a lot of motors,successfully.J

 

[matt 57]

Re: Low compression...(60/140)...Expected scope of work?

Did I miss something??? I said nothing about selling!!!

He probably misanderstood you.
I heard people use Marvel Mistery Oil to brake loose rings before starting a motor which sat for a long time but I never did that myself.
PO used the boat this season gently as he was aware of problem and told me that so this is not the case.
You think MMO will bring the compression up? It would be just quick fix for a short time. Am I wrong?
I'd rather pull the motor, take it apart and then decide what to do next.
But what is still bothering me is the fact that just one cylinder is low and easily comes all the way up after oil injection.
I read on this forum a few weeks ago that guy replaced one piston and rings and motor ran fine for a long time after that. Tryed to search that thread but with no success.
To be honest I was hoping to buy myself one season with as little investment as possible and later decide wheather rebuild or replace the motor.
There are rebuilt motors available for about $1700 with 500 hr/7 yr warranty.
Does it make sense? Rebuilt with warranty versus rebuilding one with unknown history and just rebuild kit running for about $700?
Still been thinking.

 

[45Auto]

Re: Low compression...(60/140)...Expected scope of work?

I read on this forum a few weeks ago that guy replaced one piston and rings and motor ran fine for a long time after that. Tryed to search that thread but with no success.

I may be the one that posted what you're looking for. Are you talking about post #9 on this thread?

http://forums.iboats.com/showthread.php?t=314528&highlight=piston

If so, that boat is still running fine. It was given to me because it was locked up and wasn't worth paying anyone to fix. I replaced the single bad piston that I had to break apart to get it out. Bought a set of 4 pistons with rods off ebay for $10 - I should still have 3 in the shed if you need one. I honed it and put in a new set of rings on the replacement piston. Gave it to a friend who was looking for a cheap boat to use on a small private lake.

I also replaced a single piston damaged by detonation on this motor:

http://forums.iboats.com/showthread.php?t=340492&highlight=piston

It's been running like a top since just before Labor Day.

Nothing wrong with replacing a single piston if that's all that is bad. Do you have a cylinder micrometer to check ALL the bores for roundness, taper, etc while you have it apart? How about Plastigaging the bearings and miking all the bearing journals to see what kind of shape they're in? That's good stuff to know to evaluate the condition of the motor before you dump money into band-aid (single cylinder) fixes. You probably want to know if the rest of the motor will hold up for a while. If you're looking to learn about stuff like that you've got an excellent opportunity to learn.

About the age...age wont kill a motor...owner and bad mechanic will...

You're correct, age won't kill it. But as Bond-O said, you have no idea what kind of maintenance it's had, how many times it's been overheated, etc. Age just gives people more opportunities for bad maintenance. As a general rule, a 3 year old motor will be in better shape than a 30 year old motor.

I bought her for a little money just to gain some initial experience and not going to pump a fortune in this motor, unless I fall in love with the boat after I invest some of my labor...which is possible as I like '70s.

You know your goals and what you want it for, we don't.

If you plan on keeping it, rebuild the engine while it's out if you can. If you can't afford it at the time, do what you have to. Just don't be surprised when it costs you more down the road.

 

[matt 57]

Re: Low compression...(60/140)...Expected scope of work?

Thanks 45Auto. Yes this is exactly what I was looking for. Speaking about the pistons left over, there are different engines. Or they are interchangeable?
I have no means to track down both OEM part numbers, do you? My engine ser # is at the very top of my initial post.

If you're looking to learn about stuff like that you've got an excellent opportunity to learn.

Yes, I am and I will. No special tools however, but worth to buy some. Hopefully will be able to start working on it in a month or so.
And hopefully will get help from forum comunity.
There is one issue with outdrive yet which I didn't see before I brought her home and cleaned the drive, but will post it on a separate thread.

A little bit more about engine condition:
Starts right up on land but needs cranking for 5-10 sec 2-3 times in water.
No smoke at all, just light blue for a while right after start.
Oil pressure 20 psi at idle, around 40 psi at ~2000 rpm in water.
Spark plug electrodes when initially pulled all looked the same ash-black color, dry, no rust sign.
After 20 min of idling on muffs water temp won't go above 170-175 F.
Unfortunately haven't noticed temp in water.
Compression as stated at the very top of the first post.
Runs smooth, no vibration.
Slightly noticeable tik-tik sound at the 5th cyl under valve cover.

There is one question:

At the time we tested the boat in water PO didn't want to go above around 2000-2200 rpm saying he don't want to damage the motor even more if I don't buy the boat (he told me about compression issue before).

Wasn't there something he didn't want me to discover if we ran it hard?

Thanks.

 

[havasuboatman]

Re: Low compression...(60/140)...Expected scope of work?

Did I miss something??? I said nothing about selling!!!
I gave a possible fix.I have done that to a lot of motors,successfully.J

I was making a joke Jerry,
marvel oil to fix a compression issue in a single cylinder is a temoprary fix at best. Like dumping a little sawdust into a bad automatic transmission to hide a slipping problem.

 

[Bondo]

Re: Low compression...(60/140)...Expected scope of work?

At the time we tested the boat in water PO didn't want to go above around 2000-2200 rpm saying he don't want to damage the motor even more if I don't buy the boat (he told me about compression issue before).

Wasn't there something he didn't want me to discover if we ran it hard?

Ayuh,... With all those Red Flags,... I'm just Baffled that you bought it...
There's No reason in the world you shouldn't have run it All the way to WOT....

 

[JustJason]

Re: Low compression...(60/140)...Expected scope of work?

Just alittle rule of thumb I go by,...
200psi,+ Up is a Racin' motor...
175/ 200 is a Powerfully built motor.... a Real Runner...
140/ 175 is a production motor, in reasonable to Good shape....
100/ 140 is between getting 1 more lap outa a Dead Horse, to a tired production motor....
Below 100psi Is a Dead Horse.......

My subie engine that I built 3 years ago shows 210 on my gauge You should hear the thing start on a cold morning, some people think its a diesel until it warms up a bit. I also went throught a couple of fuel pressure regulators to get it set to run its best.

when compression goes up from 60 to 140 after injecting of 10 ml of oil on the top of the piston, does it mean anything specific?

It means you probably shot 10ml of oil right up your compression gauge.

First step would be a leak down test.

But if you don't have a compressor and the tools... Pull the valve cover. Have a look at the rockers and make sure they are all the same height and a stud didn't pull. Crank it and make sure the rockers are actually moving the valves, and the valves are closing all the way. Loosen up the valves a bit and see if your compression goes up or not.

 

[matt 57]

Re: Low compression...(60/140)...Expected scope of work?

It means you probably shot 10ml of oil right up your compression gauge.

Well, I injected oil, let it sit for at least 5 min, cranked, then skrewed compression gauge...still suggesting a leak down test?
Will do on weekend anyway, just need to figure out an adaptor. I'd love to have just valve issue but I doubt...This is what I thought of first...

Thank you guys for all your suggestions, I do really appreciate it!

 

[matt 57]

Re: Low compression...(60/140)...Expected scope of work?

Ayuh,... With all those Red Flags,... I'm just Baffled that you bought it...
There's No reason in the world you shouldn't have run it All the way to WOT....

Yeah Bond-o, you might be right but...
I went to see a few before I bought this one and spent a lot of time.
Even with no experience with boats at all and little experience with motors I saw issues, bigger or smaller on every one. And I saw people were hiding even more...some didn't want to go on water...some boats were sitting for a while with no care, some people were short on time and expected a decision in less than 1hr!..some with no papers...Taking a mechanick on every trip with me to inspect them would cost a fortune even not taking in account getting on schedule with everybody.
I used to work on cars (mostly body) and can tell a lot about it and it's owner when I buy one...I thought with boats should be pretty much the same.
I bought this one and this is what I have now and willing to work on.

 

[JustJason]

Re: Low compression...(60/140)...Expected scope of work?

Well, I injected oil, let it sit for at least 5 min, cranked, then skrewed compression gauge...still suggesting a leak down test?
Will do on weekend anyway, just need to figure out an adaptor. I'd love to have just valve issue but I doubt...This is what I thought of first...

Yes i would do a leakdown. Don't always assume the worst. Start with the basics (valves) and then work your way up.

 

[jerryjerry05]

Re: Low compression...(60/140)...Expected scope of work?

I've personnaly never put sawdust in anything but leaking cracks in a wooden boat.
Marvel Mystery Oil might be a $4. fix.
I worked around saltwater for a lot of years and MMO saved a lot of motors and a lot of $$$$$.
I had some friends fix their boat and didn't follow the correct steps,MMO saved their motor when the risers on both sides started leaking saltwater into the 4 back cylinders and locked up the motor.
They called me and the motor was fixed and ran for 15yr.MMO was the only thing I did besides the risers.J