Now I know what happened -the question is WHY - 1984 115 HP I6

MortenH

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I have a rebuilt 1984 115HP I6 which ran very fine last year. This year - after my second trip out, she starts to idle rough. I checked and found that the stator was dead. Today, I replaced the stator, did a link and sync and
fired her up. She was as usual not easy to start, but not worse than normal. She ran a little rough, but it inproved after a few minutes. I let her warm up, all the time making sure she would not exceed 1500 RPM. She is in the marina, in the water, so she had plenty of water all the time. I then proceded to adjust the idle and make her run at about 800 RPM on idle out of gear. But she would nok run as smoth as before. So I decided to check if there was spark on all six - there was. All plugs are wet and
oily - all looking the same Then all of a sudden, she starts making a terrible knocking sound - kind of reminding me of two years ago when #2 piston melted. I immediately shut her down. A bit shocked, I decided to do
a compression test - all exept #3 read 120 PSI. #3 read - nothing. The needle barely moved at all. So now, the $10000 question - what happened? We have good quality fuel here in Norway, so nobody uses decarb - I am not able to get it anywere here. And there is no problem with carbon build-up in the engines. So I guess a stucked ring is not an option. Do you guys have any hints of what to do now? Do you think I have to tear her down again? Is there anyway to inspect the #3 piston / cylinder without opening her up?

-M-
 

Chris1956

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Re: What happened? I think I am in trouble - 1984 115 HP I6

Re: What happened? I think I am in trouble - 1984 115 HP I6

Mort, The number 3 cylinder is usually first to go. You can remove the transfer cover on the starboard side of the motor and see the side of the piston, rings and cylinder. As for why you were subject to the damage, it is usually overheating or a clogged carb causing a too lean mixture. If it pumps water out of the telltale on the starboard rear, and the water is lukewarm or cooler, the waterpump is probably OK.
 

Silvertip

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Re: What happened? I think I am in trouble - 1984 115 HP I6

Re: What happened? I think I am in trouble - 1984 115 HP I6

Zero compression would say there is a hole in the piston crown. Pull the spark plugs, rotate the engine by hand so each piston is at the top of the stroke and using a pen light, take a peek. Bet you have #3 with a hole.
 

MortenH

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Re: What happened? I think I am in trouble - 1984 115 HP I6

Re: What happened? I think I am in trouble - 1984 115 HP I6

Thanks alot folks.
I will check tomorrow. I will be able to get the transfer cover off without pulling the boat ashore. But if there is a hole - what could have caused it? I have a theory about a clogged #2 carburator (havent found any clogging but...), but can running lean burn a hole? The timing was set at 18 degrees BTDC - a bit under the 21 degree recomended by Clams, but that is in the "right" safe direction.

-M-
 

MortenH

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Re: What happened? I think I am in trouble - 1984 115 HP I6

Re: What happened? I think I am in trouble - 1984 115 HP I6

One more thing - would a hole in the piston make the loud knocking sound?

-M-
 

tashasdaddy

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Re: What happened? I think I am in trouble - 1984 115 HP I6

Re: What happened? I think I am in trouble - 1984 115 HP I6

possibly a chunk flying around.
 

Silvertip

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Re: What happened? I think I am in trouble - 1984 115 HP I6

Re: What happened? I think I am in trouble - 1984 115 HP I6

A lean carb and timing are two things (separately or together) that makes holes in pistons. The noise was very likely the pieces dancing around inside the engine. checking the spark plug on that cylinder will very likely show something hit the ground electrode and it may even be missing or forced into the plug.
 

AMD Rules

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Re: What happened? I think I am in trouble - 1984 115 HP I6

Re: What happened? I think I am in trouble - 1984 115 HP I6

Thats horrible. Very unfortunate. :^

Zero Compression:
Any chance you broke a connecting rod, or had a rod cap come loose (noise)? If you stick a long screwdriver in the #3 spark plug hole, is the piston moving when you turn the flywheel by hand?
 

MortenH

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Re: What happened? I think I am in trouble - 1984 115 HP I6

Re: What happened? I think I am in trouble - 1984 115 HP I6

Thanks alot. I will check all of this later today and post any results - I just have to complete 8 hours of boring work - have to get the paycheck... I am quite sure that the cooling is OK - good strong tell tale all the time.

-M-
 

MortenH

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Re: What happened? I thimk I am in trouble - 1984 115 HP I6

Re: What happened? I thimk I am in trouble - 1984 115 HP I6

OK - here is what I found. Checked first the top of the piston through the plug hole - looked fine as far as I could see. I then removed the transfer port cover and found a good inch of both piston rings gone - or at least as far as I could see through the port, and some minor parts of the piston rings was lying inside the cover. So now I am wondering what could have caused the rings to break. Is there any other possibility than breakage during mounting?

-M-
 

Chris1956

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Re: What happened? I think I am in trouble - 1984 115 HP I6

Re: What happened? I think I am in trouble - 1984 115 HP I6

Mort, If the cylinder overheats, or the cylinder to piston clearance is too great or the ridge too tall, it can break piston rings. Since you will likely need a new piston anyway, an oversize piston, rings and a bor of the cylinder are in order. You also need to check the timing, waterpump and carbs for malfuction, and fix it, or it may happen again!
 

MortenH

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Re: Now I know what happened -the question is WHY - 1984 115 HP I6

Now I have opened the beast. I found that the piston is pretty bad beaten up and must be replaced. The cylinder is fine with no scoring or marks as far as I can see. The big question is how this could happen. I am quite sure that I have enough cooling - the impeller havent run many hours. I am a bit concerned about the fuel supply though - I found that inside the fuel tank there are some rubbish, so I guess I need to clean the tank and get rid of the old fuel. Could this have been the cause.

And another thing - it is in the middle of the short boating season here i Norway and I would like to get her running again as soon as possible. Is it necessary to rebore the #3 cyl as Chris recomends? I do not have access to a micrometer to measure if the cyl is within factory specs.

-M-
 

funpilot

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Re: Now I know what happened -the question is WHY - 1984 115 HP I6

Mort,

The assumption was made that the rings may have broken up and scored the cylinder. That would make a bore job neccessary. If the cylinder and head are clean, then buying a new piston to fit the cylinder (was it already bored out some?) which would come with new rings would be fine. Usually the block will need to be honed to get the rings to properly seat. If you are serious about the short boating season, you could try installing a properly sized piston and rings, running it the rest of the season and then doing a proper rebuild. You don't have that much to lose. One word of caution though, those parts went somewhere in the engine, you need to check each cylinder for scoring, and fix as needed. You will also have to make sure the cylinder head is smooth, and not nicked up.

Good gas can still carbon up under the right conditions of heat, timing and carburation. There are a variety of ways to decarbon. You should consider investigating them.

The rest of the pistons should be checked to see if any of those rings are starting to stick. If they are, then you need to free them up before you reinstall them.

Finally, check your fuel system and carburetors carefully. Something is amiss, but I agree with you, it probably isn't the impeller.

good luck,
fp
 

MortenH

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Re: Now I know what happened -the question is WHY - 1984 115 HP I6

Thanks alot. Sounds like I have a plan. It is as far as I can figure out a standard sized piston, but I have to check that again to be 100% sure. I am beginning to think that maybe I will rebore all the 5 remaining cylinders when the season is over ( mid september). #2 was rebored 2 years ago when the piston melted completely due to fuel problems.

-M-
 

Clams Canino

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Re: Now I know what happened -the question is WHY - 1984 115 HP I6

Whatever happened did not happen whil eit was warming up out of gear. It prolly happened that 1st trip out and you didn't know it till it finished dying.

And Silevertip... you've never seen a Mercury direct charge piston and are SPECULATING. They literally can't burn holes in the crown due to the funky design, they just plain melt down all over and stop working.

Iether way it's gotta come apart. No reason to overbore the ones with no damage, just re-ring them.

-W
 
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