New owner of 115 merc, '87: rattle sound when running

[chrisg]

Hello;

I just got a 24-ft 'toon with this merc on the back. It appears to have power, but I noticed that when running, there is a rattling sound (and changes intensity at times), it?s not when moving slowly, or if I reeve the engine in neutral. If it were a 4-stroke, I would say it sounds like valves. Could the shaft / lower unit gears be loose/worn and wobbling? (or would that prevent the thing from running at all).

It appears to run OK, other then this rattling sound. (I do recall the Mercs of that time were loud- we had one when I was a kid ? late 70?s) I looked inside, nothing loose, but noticed that the insulation on the cowl is missing. When you put you hand on the cowl when running, you can feel the rattle vibrations.

If there were issues with the lower unit, would I experience other problems? (Like I say, it has power, and moves the ?toon at a good clip- 20MPH 5000RPM WOT). Don?t know the prop pitch. And its missing that fin that sticks down behind the prop. It?s a brass prop I think.

Could it be the insulation is missing, and it?s the cowl that is rattling?

I also go through 5 gal of gas traveling 17Miles. And that?s at ? throttle! ? I?m getting a bigger tank!

 

[outhere99]

Re: New owner of 115 merc, '87: rattle sound when running

How about the cowl? Mine is loose and will rattle...unless I rig here and there.

Mercury has one of the most ridiculous set ups for such a simple chore (on earlier engines). 4-6 pieces of hardware to cover an engine, and not very well.

If that's not it I'd pull the lower unit and check there. Replacing the whole unit instead of a few bearings could cost big! Check the gear oil and check for metal. If it's rattling that bad I'm sure you'd find some.

How about the steering and throttle rigging? Have you looked there?

 

[j_martin]

Re: New owner of 115 merc, '87: rattle sound when running

That engine fully advances the timing at less than half throttle. It's conceivable it's a bit early.

Try a pony tank with 92 octane gas in it for a trip. If it greatly lessens the noise, then that's the problem. Fix is to a. go to 92 octane, or b. retard max timing a bit. or c, like I do, only run it at idle and Wide open.

hope it helps
John

 

[chrisg]

Re: New owner of 115 merc, '87: rattle sound when running

Thanks for the responces. I was thinking, it may be the cowling, as I stated, it is missing insulation, and I did not check if ALL the parts are tight! I wonder if adding insulation ( if possible) would help reduce the noise.

Also, I never thought about this; Im running the corn-gas( Ethol) in it, perhaps thats a No-No for the older motors.

Well, I did take it to the marina last night, let them look it over. I'll provide update when I get a verdict on it.

 

[j_martin]

Re: New owner of 115 merc, '87: rattle sound when running

It'll theoretically run the gasohol ok. The problem is that if it gets any water in it, the water and alcohol seperate out. That can cause both short term and long term problems.

I use 92 octane non-oxygenated (no alcohol) fuel. Eliminated a ton of grief. Any little bit of water that gets into the fuel, such as when some dork goes by my bass bote with a 30 foot cruiser at half throttle and durned near swamps it, is picked up by the fuel/water seperator.

To put it another way. 5 gallons of gasohol has a half gallon of ethanol in it. Add a few ounces of water, and you drop out a half gallon of alcohol/water that doesn't run the engine. Without the ethanol, you have a few ounces of water to contend with.

About the noise. The engine is pretty noisy with the covers off. I imagine having the foam missing could make it unusually noisy.

hope it helps
John

 

[gregpro50]

Re: New owner of 115 merc, '87: rattle sound when running

Mine rattles too with 87 octane with 10% ethanol. I will never run that stuff again. Now I run the 93 octane and it runs nice and smooth.

 

[chrisg]

Re: New owner of 115 merc, '87: rattle sound when running

thanks all for your responces. I heard back from the marina, they said the motor is no good. it has low compression on 3 of the 6 cylenders. the rings are bad and the insulation is falling of the wiring. I should look for a newer one.

It does run and appears to have power, just goes through gas like crazy. Well, this would explain why I cant get it to go faster then 18MPH, running at 5000RPM WOT. I guess the rattling is the pistons.

Well, Guess I now have a 24-ft 'toon with no motor. thats what you get when you buy a cheap boat!. So after getting new seats and a motor. It will cost me 8K! well the carpeting is new and the deck is solid. with no dents.

Anyone know where I can get a good used Merc? ( Then I dont have to change the cables)