Merc 125 making funny knock at 4000+ rpm

[j_martin]

Re: Merc 125 making funny knock at 4000+ rpm

Piston slap in a two stroke? It's more like bearing rattle caused by the higher compression when the water enters. Water is a uncompressible liquid which is why it's not a good thing to have in your combustion chamber.
QUOTE]

Sorry to say it as we've had different options on a few threads but I don't agree with this at all......

When you compress water it turns into Hydrogen and oxygen, the extra gasses cause your combustion to burn too hot which then in turn can cause the piston crown to melt, causing detonation and worse if left unchecked. And i've rebuild my 1984 115hp I-6 due to intermittent piston slap- half the top ring broke (bad ring pin) causing un-even pressure on the top side of the piston which pushed the bottom part against the wall every now and then. I highly doubt that that noise is from bearing rattle, if it is his bearings are shot.

I don't know where you learned chemistry, but this is B as in B, and S as in S.

 

[j_martin]

Re: Merc 125 making funny knock at 4000+ rpm

Parts are ordered, again - weather willing - should have the boat on the water tomorrow afternoon.

Maybe he hasn't seen it, but a lot of folks here have. I think yer good to go.
Clean piston, dripping water, bad gaskets, all the ducks are in a row.

On the noise, you're probably hearing noise that most folks wouldn't notice. That's a good thing. In this case it probably saved you a lot of trouble with eroded pistons, stuck rings, damaged needle bearings, etc.

John

 

[sixstring531]

Re: Merc 125 making funny knock at 4000+ rpm

John -

I am a self proclaimed home mechanic for the cars/trucks (couple years of auto shop in highschool - when they still offered it ) and am always paranoid of funny noises. I want to hear harmony when a motor runs and when something disrupts the rhythm, there has to be some sort of a problem.

So again folks, thanks for the advice, we'll see what today brings.

 

[arsenalpsu]

Re: Merc 125 making funny knock at 4000+ rpm

I wasn't disputing the cause of the problem, just the reason for the noise. But it sounds like you fixed it and hopefully the knock won't come back. If it does I'd say sure it's piston slap....but lets not think about that !! Good luck!

PS: the mention about the heat/pressure causing small amounts of water to vaporize adds extra oxygen & hydrogen to your fuel/oil mix, causing a hotter burning and leaner mix. The extra gasses also add volume and create higher PSI in the cylinder which puts added stress and heat on the crow. (merc went to low dome pistons to lower the PSI and therefore the burn temp in later inline models to try and extend the life of pistons as the newer crappy gas has to burn hotter) It won't cause damage right away but if left unchecked can cause a meltdown in that piston. It?s not just the pressure alone?.. I guess I should have explained my self better.

 

[sixstring531]

Re: Merc 125 making funny knock at 4000+ rpm

So let's say it doesn't fix it and it's piston slap -- what is the course of action here?

 

[arsenalpsu]

Re: Merc 125 making funny knock at 4000+ rpm

Well I would try some decarb to make sure it isn't a sticking ring causing uneven pressure on one side of the wall, if knocking still continues and it isn't taken care of it could cause you to throw a rod making your block scrap metal. The only option would be to split the block and replace that piston. You'd have to check for scratches and uneven ware in the cylinder wall. I'd also replace all the rings while I was in there.

There is a lot more to it BUT other guys that know these engines getter than I say you've fixed it so run the snot out of her unless you hear more knocking!

Happy boating

 

[sixstring531]

Re: Merc 125 making funny knock at 4000+ rpm

Hope it does fix it. Looking forward to running without noise, but getting fish in the boat is the only real reason I put it in the water

 

[j_martin]

Re: Merc 125 making funny knock at 4000+ rpm

I wasn't disputing the cause of the problem, just the reason for the noise. But it sounds like you fixed it and hopefully the knock won't come back. If it does I'd say sure it's piston slap....but lets not think about that !! Good luck!

PS: the mention about the heat/pressure causing small amounts of water to vaporize adds extra oxygen & hydrogen to your fuel/oil mix, causing a hotter burning and leaner mix. The extra gasses also add volume and create higher PSI in the cylinder which puts added stress and heat on the crow. (merc went to low dome pistons to lower the PSI and therefore the burn temp in later inline models to try and extend the life of pistons as the newer crappy gas has to burn hotter) It won't cause damage right away but if left unchecked can cause a meltdown in that piston. It?s not just the pressure alone?.. I guess I should have explained my self better.

When you vaporize water, it does not break down into hydrogen and oxygen, it just consumes a large amount of heat vaporizing.

There's been much experimentation with water injection, or water purposely left in alcohol fuel. The water, because of the phase change thermodynamics, does add to the volumetric efficiency, but we found that peak cylinder pressures and temperatures were lowered a bit. The deal killer is dealing with freezing, and high ambient temperature operation. With the alcohol/water fuel, poor lubrication, and back then the inability to suspend a decent and economical lubricant killed the concept. Castor oil worked, but it has many drawbacks, like being expensive and poisonous.

The reduced octane in fuel has to do with purposely reducing the combustion chamber pressures to limit NOX emisions by lowering compression or remixing exhaust gasses in the fuel mixture. Such an engine does not benefit from ultra high octane. The other item is the most economical way to achieve the high octane was tetraethyl lead, which of course is a serious systematic and environmental poison. The oportunity to improve refinery economics was not lost, either.

On the noise issue, it is not uncommon for normal bearings to make noise under particular conditions. The most obvious is automobile engine bearings that will "sing" a bit on startup until they are pressurized with oil. I've heard that noise in engines that ran another 100,000 miles after I heard it. The bearings are not out of tolerance, but maybe near the outside limit.

The bearings in an outboard 2 cycle are never pressurized.

hope it helps
John

 

[j_martin]

Re: Merc 125 making funny knock at 4000+ rpm

So let's say it doesn't fix it and it's piston slap -- what is the course of action here?

Patience grasshopper. This is a serious problem, and it probably will cure the noise.

 

[sschefer]

Re: Merc 125 making funny knock at 4000+ rpm

Patience grasshopper. This is a serious problem, and it probably will cure the noise.

John - http://www.boats.net/parts/detail/merc/M-9267.html Thought you might be interested.

 

[sschefer]

Re: Merc 125 making funny knock at 4000+ rpm

So let's say it doesn't fix it and it's piston slap -- what is the course of action here?

Let me see how close I am at this -

First thing in the morning, you can run it up to 5000 RPM and there's no noise. If you make a quick run down the lake, about 10 minutes into it you start hearing the noise so you back down to around 3800 and it subsides. The rest of the day, you can't get over 3800-4000 RPM without hearing the noise.

The exhaust setup consits of a gasket, divider plate, gasket and cover. If your mechanic does not check for a warped divider plate and it's warped, the problem will come back. How long it takes to come back varies. Mine would last about a season.

That's what will throw you into a troubleshooting nightmare. You think you fixed it and it comes back so you start loooking for all sorts of other causes. Don't get off track, just order a new divider plate.

 

[sixstring531]

Re: Merc 125 making funny knock at 4000+ rpm

I made sure to ask him to check the plate for warping. He said he was taking it to his other shop to really clean it up and he would definitely know if it was warped by cleaning it up.

The gaskets are about $40 each (expensive gaskets) - though I suspect a new engine costs a lot more than that.

 

[j_martin]

Re: Merc 125 making funny knock at 4000+ rpm

John - http://www.boats.net/parts/detail/merc/M-9267.html Thought you might be interested.

Used?. or NOS very limited in quantity. GLM is still listed in catalog as TBA.

 

[sschefer]

Re: Merc 125 making funny knock at 4000+ rpm

Used?. or NOS very limited in quantity. GLM is still listed in catalog as TBA.

Just sent you an email. Not sure. I couldn't get six so I ordered the complete kits, Thought I'd leave them for you if you wanted them.

 

[sschefer]

Re: Merc 125 making funny knock at 4000+ rpm

I made sure to ask him to check the plate for warping. He said he was taking it to his other shop to really clean it up and he would definitely know if it was warped by cleaning it up.

The gaskets are about $40 each (expensive gaskets) - though I suspect a new engine costs a lot more than that.

Sounds about right, you should get two sets. They come with a very explicit guide about checking the diverter/divider part number and using the correct gasket.

Looks like your on your way to having a good running motor.

As a side note, if you run a vented prop with these engines, when the two injectors and accelerator pump kick in it will feel like you hit the after burners.

 

[sixstring531]

Re: Merc 125 making funny knock at 4000+ rpm

Well, new gaskets on, apparently runs a lot better (didn't run bad to begin with -- just that noise) but noise still happens. I am taking it somewhere else. I found out the grandfather of the owner of this place won't even take his boat there. - kind of found out after they had my boat that their reputation is horrible.

Called a couple reputable places - here are some theories:

1. Bad bearing(s) - though both reputable places don't think so since it only happens around 4000 rpm and when powering down

2. Reed valve sticking/breaking

3. Ignition electrical problem (from the highest recommended place)

I'm not sure I have the batteries (2 deep cycle one starter) hooked up correctly ( the bad marina did it for me) - running the trolling motor on 12 runs it stronger than on 24 and I think it should be the other way around. Also, when i plug the charger in, it trips my wall breaker think on the plug - I was told I probably need to unplug my trolling motor, which I have not tried yet - that or it's hooked up bassackwards. This might explain an electrical prob of some kind. I was having problems with the fishfinder just turning off and not being able to turn on again for a while - not sure what that's all about.

Bad or not-so-reputable marina says the cylinders and pistons look great, no scoring, not dirty and can still see the mill marks (it has great compression in all cylinders) and rings look great.

Anyway, so the problems (and there is no way they can know until they take it on the lake, as it doesn't do the sound on land) could be 1. Bearing(s) 2. Reed Valve? 3. Electrical in nature.

What you think? I hate the crap out of this.

 

[sschefer]

Re: Merc 125 making funny knock at 4000+ rpm

Well, new gaskets on, apparently runs a lot better (didn't run bad to begin with -- just that noise) but noise still happens. I am taking it somewhere else. I found out the grandfather of the owner of this place won't even take his boat there. - kind of found out after they had my boat that their reputation is horrible.

Called a couple reputable places - here are some theories:

1. Bad bearing(s) - though both reputable places don't think so since it only happens around 4000 rpm and when powering down

2. Reed valve sticking/breaking

3. Ignition electrical problem (from the highest recommended place)

I'm not sure I have the batteries (2 deep cycle one starter) hooked up correctly ( the bad marina did it for me) - running the trolling motor on 12 runs it stronger than on 24 and I think it should be the other way around. Also, when i plug the charger in, it trips my wall breaker think on the plug - I was told I probably need to unplug my trolling motor, which I have not tried yet - that or it's hooked up bassackwards. This might explain an electrical prob of some kind. I was having problems with the fishfinder just turning off and not being able to turn on again for a while - not sure what that's all about.

Bad or not-so-reputable marina says the cylinders and pistons look great, no scoring, not dirty and can still see the mill marks (it has great compression in all cylinders) and rings look great.

Anyway, so the problems (and there is no way they can know until they take it on the lake, as it doesn't do the sound on land) could be 1. Bearing(s) 2. Reed Valve? 3. Electrical in nature.

What you think? I hate the crap out of this.

This time, don't tell them anything, make them figure it out. Let us know what it was. The reason they can't do it on the hose or in a tank is because they can't make enough water pressure.

Like I said, don't tell them and don't pay them for all the junk they do trying to find the problem, that includes parts! My bet is that the divider plate is warped and the guy never really checked it. Or... He put the wrong gasket set in.

 

[sixstring531]

Re: Merc 125 making funny knock at 4000+ rpm

This time, don't tell them anything, make them figure it out. Let us know what it was. The reason they can't do it on the hose or in a tank is because they can't make enough water pressure.

Like I said, don't tell them and don't pay them for all the junk they do trying to find the problem, that includes parts! My bet is that the divider plate is warped and the guy never really checked it. Or... He put the wrong gasket set in.

I think I will let another marina figure it out. I will probably mention this too them though - or else they might not check it? OR - would a knowledgeable mechanic know what the sound is when they hear it on the water?

 

[sschefer]

Re: Merc 125 making funny knock at 4000+ rpm

I think I will let another marina figure it out. I will probably mention this too them though - or else they might not check it? OR - would a knowledgeable mechanic know what the sound is when they hear it on the water?

A knowledgable mechanic is going to go right to checking the plugs and piston tops. When he see's how clean they are he'll know where to start looking.

I almost burst out laughing when you said the other mechanic remarked how clean the cylinders were... Yeah, water does that...

 

[sixstring531]

Re: Merc 125 making funny knock at 4000+ rpm

schaefer:

Where you at? Can I just bring it to your house?