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86 merc 115 inline 6 sudden RPM drop and recovery where do I start

J

jon

Regular Contributor
" Hi,

This season I pu

" Hi,

This season I put around 100 hours on my 86 merc 115 inline 6; a total of four times in those hours this has happend, and I don't know where to start looking for the problem.

At near full throttle (5000 RPM or so), after running for a period of time, the RPM's will drop suddenly (mabye down to 3000). When this happens I allways cut it down to idle, shut it off, pull up the prop and take a look at it expecting to find that either the prop has hit something, or that there is some junk wound around it, but it has allways looked fine. It then will start right back up and run as if nothing happend.

I have had the prop off and there is nothing wound around the prop-shaft; the spark plug wires look new, Does anyone know what is likely to cause this, or where I should be looking?

Thanks,

Jon "
 
"Jon,

It sounds like an ign

"Jon,

It sounds like an ignition problem. It could be one switchbox cutting out or a stator that is on the way out.

If it happens again try to determine where the source is, by removing the spark plug wires one at a time and noting if some don't have any effect when removed. Use an insulated tool for removing the wires.

You might be able to tell if the problem is isolated in one switchbox this way.

If you suspect the stator, you could test the resistance in the two windings at different times, especially when the problem arises. This is difficult on these engines because it takes some time to remove the wires to perform the test, and this gives the windings a chance to cool down a bit. You might notice that the resistance measures erractly from time to time, and this would point toward the stator.

"
 
"Jon,

It sounds like an ign

"Jon,

It sounds like an ignition problem. It could be one switchbox cutting out or a stator that is on the way out.

If it happens again try to determine where the source is, by removing the spark plug wires one at a time and noting if some don't have any effect when removed. Use an insulated tool for removing the wires.

You might be able to tell if the problem is isolated in one switchbox this way.

If you suspect the stator, you could test the resistance in the two windings at different times, especially when the problem arises. This is difficult on these engines because it takes some time to remove the wires to perform the test, and this gives the windings a chance to cool down a bit. You might notice that the resistance measures erractly from time to time, and this would point toward the stator.


"
 
" Tony,

Thanks, well

" Tony,

Thanks, well that's a place to start; It's not quite practical yet to try and catch it while it's down, it bounces back so fast, I think it goes away before I even have a chance to shut it down. This sounds a little funny, but hopefully it will get worse so I get a better shot at figuring out what it is.

There's one other thing I probably should have mentioned because it could say something about the electrical condition; once the tach stopped working for 20 minutes, connections seemed fine, and then it just fixed itself. Has never happend again.

If we are talking about a stator, or a switchbox is this the type of thing that will allow me to limp home when it finally breaks, without doing serious damage to anything else? Or is it going to leave me stranded?

How many hours do switchboxes and stators usually last? I think the stators are a little pricey ($250 range sound right? ), how about the switchboxes?

I bought this motor about a year ago, put around 100 hours on it this season, compression is about 125-130. I rebuilt the carbs and fuel pump in the spring, and replaced an o-ring in the power-tilt piston. Has been very reliable ever since.

I've thought about upgrading to something newer, and more "reliable", but after the season on this engine, I don't know if a brand new one would be worth it for the money.

There are two things it needs fixed, one is the thing I described before, the second is a busing or bearing where the lower motor mount fits around a grooved cylinder that allows the outboard to turn. It has gone to hell, and the motor has some play back on forth; If you know what problem I'm talking about, It's been like this since I've had it, I can't notice it when It's in use; Is it worth fixing? I think it might require pulling the motor off the bracket that secures it to the transum.. Have you ever seen one of these go? Any thoughts?

Thanks,

Jon "
 
"Jon,

The tachometer proble

"Jon,

The tachometer problem suggests that the stator may be the source of the trouble, because the tach runs off of the stator.

The lower mount is quite an undertaking. You have to take just about everything apart to repair it.

"
 
" Tony,

Thanks, I gues

" Tony,

Thanks, I guess that makes the stator my #1 suspect; About the lower motor mount (the bushing/bearing or whatever it is that needs replacement there), will the play in it destroy anything else, or is it just a performance issue? I'm trying to figure out if it's worth replacing, I doubt I'll run this engine for more than another couple seasons.

Jon "
 
"Jon,

I have seen people ru

"Jon,

I have seen people run with a damaged lower mount on these motors for years.

The problem that usually arises is that the engine begins to contact the transom when in forward gear. When it gets to that condition, you have to address the problem.

"
 
" Tony,

Thanks again

" Tony,

Thanks again, one more thing I'm thinking about; I'll be laying up the boat for the season, which means I'll be looking into the stator soon. If it tests fine, I'm wondering if I should just go-ahead and replace it anyway, or if I should wait until it breaks.

What I need to know is if I'll be able to limp home with a faulty stator, and if it will destroy any other electrical parts.

Thanks,

Jon "
 
"Jon,

If the stator is malf

"Jon,

If the stator is malfunctioning it could lead to other problems, but it is difficult to predict exactly what they may be. I would suggest that you hunt around for a used stator and install it. Then you should be able to decide over time if the problem has gone away, and it won't cost much.

"
 
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