Merc 125 making funny knock at 4000+ rpm

[sixstring531]

My 97 Merc 125 Outboard has been making a quick "ticking in succession" (like a quick burst on a machine gun, but not nearly as loud) when running on the lake at 4000+ rpm. It is intermittent and when it happens, you can feel a slight loss of power while it happens. It only happens for about 1/2 a second then resumes running fine, then might do it again a minute later....sometimes didn't do it at all. It seems to do it more the faster you go or higher on plane you get.

The boat is now at the marina being looked at (which is not on the water) and they are trying to figure out a way to get it to make the sound under load because it won't do it in neutral.

Recently had a rebuilt lower unit put on (seems to be running fine), new control box and carbs (all 4 of em) cleaned.

Compression on all cylinders (from top to bottom) 110, 110, 111, 110

Any suggestions?

Thanks fellas

 

[sschefer]

Re: Merc 125 making funny knock at 4000+ rpm

That's just about the point where detonation might occur. The symptoms sound similar also. Have them look for aluminum deposits on the plugs or a piston top that looks like it's pitted or carboned up. Sometimes the black soot disguises the pitting. You did the right thing by getting it in to be looked at. I congratulate you!

 

[sixstring531]

Re: Merc 125 making funny knock at 4000+ rpm

What would be course of action for a pitted piston? I should also mention it does bog a tad on the take off -- kind of bogs then jumps up in power (it is the 2+2 I think) -- so don't know if I need to change any fuel filters. It is 13 years old and I am unsure if any have been changed.

I think they put all new spark plugs in it recently because when we put the newer lower unit on and took it out with full gas it wouldn't even accelerate above a slow move - marina said spark plugs (since they were original) so they said they changed em and now it runs great except the sound i described.

I hate spending money on this crap.

 

[sschefer]

Re: Merc 125 making funny knock at 4000+ rpm

What would be course of action for a pitted piston? I should also mention it does bog a tad on the take off -- kind of bogs then jumps up in power (it is the 2+2 I think) -- so don't know if I need to change any fuel filters. It is 13 years old and I am unsure if any have been changed.

I think they put all new spark plugs in it recently because when we put the newer lower unit on and took it out with full gas it wouldn't even accelerate above a slow move - marina said spark plugs (since they were original) so they said they changed em and now it runs great except the sound i described.

I hate spending money on this crap.

Oh shoot I didn't know is was a 2+2. That nois is water intrusion comming from bad gaskets in the exhaust divider on the port side of the engine. It's the plate the covers the length of the motor side and has about 20 bolts in it. Remove it, remove and clean the divider plate, replace the gaskets and you'll be back in business. Easy job.

As for the bog, that is somewhat normal but it can be exagerrate by a dirty filter on the injector side. That motor has two mechanical injectors on the sides of cylinder 3 and 4. To change the filter you have to remove the oil tank. You'll see the filter when you do that. To clean the injectors run some SeaFoam in your gas and if that doesn't do it, you have to remove the electrical plate on the stbd side of the engine to get to them and manually clean them.

 

[sixstring531]

Re: Merc 125 making funny knock at 4000+ rpm

Funny you mention that plate -- and you sound pretty sure of yourself here (I just called my marina and told them, they are going to check it out) -- when we had the cover off at the marina and were running it with the rabbit ears (first test runs) I noticed water trickling down the plate when he would gun it.

This may be a leak you say? So water is getting into the cylinders? I suppose this might explain the intermittents of the sounds.

Thanks for the help man -

Richie
417-773-6474

 

[arsenalpsu]

Re: Merc 125 making funny knock at 4000+ rpm

How do you know that it isn't piston slap??

 

[sixstring531]

Re: Merc 125 making funny knock at 4000+ rpm

How do you know that it isn't piston slap??

This is what my mechanic thought it might be - and they are working on a way to get it to make the sound at the shop by purchasing a larger tank - since they can't rightly get it up to 4000+ on the water and work on it at the same time.

What do you do for piston slap?

 

[sschefer]

Re: Merc 125 making funny knock at 4000+ rpm

How do you know that it isn't piston slap??

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.

I've owned them and know that they are prone to this problem and that it always occurs between 3600-4000 RPM. I made changing out the gaskets (30 min job) a part of the annual tune up process.

Make sure to have the Mech follow the Merc/Mariner Service Manual and check the Divider for warping. The cover will suck down but if the plate is warped the problem will return. Also make sure he orders the gaskets for the correct divider plate. There were two styles he needs to reference the number on the divider plate when ordering.

As a side note, most sets come with both style gaskets.

 

[sixstring531]

Re: Merc 125 making funny knock at 4000+ rpm

Because I've owned them and know that they are prone to this problem and that it always occurs between 3600-4000 RPM. I made changing out the gaskets (30 min job) a part of the annual tune up process.

Sounds good man - I told the mechanic and he is going to check it out.

 

[j_martin]

Re: Merc 125 making funny knock at 4000+ rpm

How do you know that it isn't piston slap??

Piston slap would not be intermittent.

Water intrusion should be pretty apparent when you look in the cylinder. It'll be squeaky clean all over, instead of a systematic carbon deposit on the piston.

 

[sschefer]

Re: Merc 125 making funny knock at 4000+ rpm

Piston slap would not be intermittent.

Water intrusion should be pretty apparent when you look in the cylinder. It'll be squeaky clean all over, instead of a systematic carbon deposit on the piston.

Yep, Inline motors (particularly sixes) are the most harmonious engines built. They rarely slap pistons. Even the 60 deg short stroke V's don't do it much. They do scuff them up but that's usually due to swelling caused by detonation or high cylinder temps.

Roller bearings do rattle and that's the noise that's being heard in this engine. It's an odd noise the first time you hear it and a weird one to diagnose.

 

[j_martin]

Re: Merc 125 making funny knock at 4000+ rpm

Yep, Inline motors (particularly sixes) are the most harmonious engines built. They rarely slap pistons. Even the 60 deg short stroke V's don't do it much. They do scuff them up but that's usually due to swelling caused by detonation or high cylinder temps.

Roller bearings do rattle and that's the noise that's being heard in this engine. It's an odd noise the first time you hear it and a weird one to diagnose.

Let me guess why. When a little water gets into the cylinder, and it misses, there's a little extra extension force on things at the top of the piston throw because there's no combustion pressure to keep it tight. If a bearing has any lash, even if it's within tolerance, it can make some sound.

Anyway, it's stacking up to looking at the exhaust chest like was said.

 

[sixstring531]

Re: Merc 125 making funny knock at 4000+ rpm

Mechanic just called and said gasket was blown in a couple spots! Thanks iboat people! He's ordered the gaskets and is going to clean up the parts (which he says aren't warped) and will put them on tomorrow and hopefully get it out for a test run - weather permitting.

Hopefully (mechanic is pretty confident....though he said he's never encountered this problem) this clears it up.

 

[arsenalpsu]

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.

 

[sixstring531]

Re: Merc 125 making funny knock at 4000+ rpm

If the bearings are shot, wouldn't it pretty much do it all the time? The fact that one member told me what he thought it was and when checked proved to be what he was talking about leads me to believe I am on the right track.

The motor runs strong, runs great, great compression...just had that intermittent sound on occasions so I took it in. Keeping my fingers crossed and hoping this is all it needs.

 

[arsenalpsu]

Re: Merc 125 making funny knock at 4000+ rpm

If the bearings are shot, wouldn't it pretty much do it all the time? The fact that one member told me what he thought it was and when checked proved to be what he was talking about leads me to believe I am on the right track. .

I'm not saying your bearings are shot, just saying i think that noise was piston slap from overheading due to the extra gasses in the cylinder during ignition. This causes your piston to expand slightly, if it didn't do any real damage you're prob ok, you might have some piston scuffing. If there was enough room in your bearings to make that noise wouldn't it always do that, water or not? Did your guy look at the top of the pistons?

 

[sixstring531]

Re: Merc 125 making funny knock at 4000+ rpm

I think he did and said it looked pretty clean (like another member stated above if water is getting into it) and he thinks changing the gasket will fix it 100% (though again, he said this is the first he's ever seen do this)

 

[sschefer]

Re: Merc 125 making funny knock at 4000+ rpm

Let me guess why. When a little water gets into the cylinder, and it misses, there's a little extra extension force on things at the top of the piston throw because there's no combustion pressure to keep it tight. If a bearing has any lash, even if it's within tolerance, it can make some sound.

Anyway, it's stacking up to looking at the exhaust chest like was said.

Yep, it took me a long time to understand it, I was thinking expanding steam higher compression but what actually happens is very similar to having a coil double fire.

 

[sschefer]

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.

You're welcome to your opinion and I fully see why you've formed it. Let's see what the outcome is and compare notes at the end.

 

[sixstring531]

Re: Merc 125 making funny knock at 4000+ rpm

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