1994 Mariner 75 Hp 3 Cylinder Issues ??????

[chadpcb]

I will try to discribed the issues to the best i can. I have never owned a mariner before so I do not know if the sounds should be there or not.
Issue #1
There is a top end knock. I have used a scope in order to try and locate it but I can not. It seems to be coming from under the fly wheel. I have scoped every cylinder thinking it is a bearing or a bolt in a piston arm but it is not coming from any of the cylinders that I can tell. It also seems like when you when you apply throttle it doesn't get any louder and seems to go away.
Issue #2
I throttled up on the back side of a wave and the engine just reved up. Pulled back to nuetral and back into gear again and the engine responded as normal.
Any ideas and any information would be greatly appreciated....
Thanks

 

[Texasmark]

"I throttled up on the back side of a wave and the engine just reved up." That sounds like a spun prop. Noises in metallic devices can be deceiving. If your prop has been slipping and catching without you realizing it, your engine may have realized it and the load/unloading of the geartrain may be what you are hearing.

Scribe a line from the brass spline of your prop to the outer hub material be it alum or SS. Run the prop hard. Check your line. If not as you marked it get your hub repaired or a new prop.

Mark

 

[chadpcb]

Ok sorry for the delay been busy. Update on both issues. The prop is not slipping it was a cavitation is in the rough water. (Pontoon boat) Now after coming in the other night and when flushing the motor I noticed that if I put my ear to the center section of the motor the section between the foot and and the top section of the motor. The knock is louder in this section especially to the exhaust side of the and not the shaft side. Hope this helps a little more.

 

[enginepower]

Knock "inside" motor??? well.....that means it needs to be rebuilt before pieces come flying out of it...

 

[chadpcb]

The issue is there is nothing knocking in the motor. I have used a stethoscope to check each cylinder, connecting rod, and bearings. It does not seem that the knock is coming from the block itself. I can say this what ever is causing this noise it is a constant sound it doesn't increase or decrease with throttle. But however it does seem to stop when more throttle is applied. It doesn't make sense

 

[Taxus812]

So the frequency between knocks does not increase with RPM? When in gear does it change with RPM ?

 

[chadpcb]

After spending the majority of the day on the water I have found that. When throttle and gear is engaged the knocking stops at about 1300 rpms.

 

[Texasmark]

I'm going to say it's Clutch dog rattle and is caused by pulsations of the prop. I have it and it is prop related. On me I get it when I run my heaviest, highest pitched prop. Once the water pressure against the prop stabilizes at the higher rpms, it goes away. Merc says it's nothing to worry about and if you ever tried to machine a lower unit gear, you know just how hard machined-treated gear teeth are.

It's a PIA for me when loading on the trailer especially and if I don't want to hear it I have to shift between N and F to get lined up at the right speed. When I hear it leaving the dock, I just up the throttle a bit till it stops. But I like the 24P prop and will run it and will tolerate the noise. Been that way for 4 or 5 years and every year when I change fluids in the LU, there is minimal metal on the magnetic insert in the drain plug and the oil is blue-black (Quicksilver premium LU oil).

Mark

 

[chadpcb]

Alright actually had time to snatch the LU off. Ran the motor like that and the tick, knock what ever it maybe is still there. It almost sounds a valve ticking on a car engine. I have isolated the location to the area behind the oil tank between the 2nd and 3rd cylinder. This is where it is loudest besides the exhaust mid section.

Things I have done.

1. Removed LU ran engine..
2. Removed all spark plugs and checked for any play in the pistons. No scaring on walls of cylinder.
3. Run scope all over motor to isolated the loudest area

What I have found is that it is the loudest in the mid section of engine and behind the oil tank between the 2nd and 3rd cylinder....

 

[chadpcb]

I am thinking of decarbing the engine as I don't know if it has ever been done.