Fluctuating RPM on 96 Mariner 115

[pinellas50]

Re: Fluctuating RPM on 96 Mariner 115

Took my dad out after the King fish today. I had extended run times out and back and had to stay down around 3800 rpm most of the time so I wasn't jumping swells. I was able to run 4400 rpm for a little while on the way back.

Start up, idle, and the transition from 2 to 4 cyl were all spot on today. While cruising, that intermitent "miss" I was talking about was present. It would come and go a few times every minute. The rpm's never dropped when it did this. You could feel and hear when the miss occurred. It only did the fluctuation thing once at 4200 at which point the rpm's dropped down to 4000. That rpm drop was almost instant, not a bog like starving for fuel. The miss was sometimes combined with a knocking or rattling sound.........I don't know how to describe it.............and the motor has always made this sound, from time to time, when quickly backing out of the throttle(but not going straight to the idle position) from cruising rpm. In other words, the sound is familiar to me as it happens every now and then when decelarating.(it made this sound back when the motor was running great without any type of issues)

So obviously something is still not right. And I am still thinking electrical. But I am at a loss as to what it may be or where to go from here. Aside from the "run it till it breaks then you know" approach. Have I confused everyone at this point?

 

[randyrandy]

Re: Fluctuating RPM on 96 Mariner 115

I had similar experiences with both an Erude 140 hp and my present 150 Mariner/Merc. It turned out that replacing the switchboxes cleared it all up. (called power packs on Erude/Johnson.) Don't know if this will help you, but have you tried replacing the powerpack/switchboxes. One other thing, could the coil to sparkplug wires be leaking spark due to poor insulation, as that happened to me before and caused a similar problem. Are your coils good? Good luck.

 

[CharlieB]

Re: Fluctuating RPM on 96 Mariner 115

What spark plugs are you using?

Look closely inside them, can you see a black 'ring' up inside the center porcelin? how far up is it?
too near the tip and the plug can short and misfire, indicating it is time for new plugs or possibly a change to a '7' heat range if you usually operate the motor at lighter loads.

If memory serves, my 96 115 called for NGK BPZ8-10 which costs quite a bit more than the BPR8HS, set gap to .040

 

[pinellas50]

Re: Fluctuating RPM on 96 Mariner 115

Charlie, the plugs are new. I put them in some time around January while doing all of the other stuff I tried. The plugs are NGK BP8H-N and there is a "10" under that number. The folks at the marine store looked them up and sold me what the book said the motor should have......according to them. I gapped them at what the book said and I don't remember what that value was at this point. This problem has been present on both sets of plugs.

I have the cowl off and was just about to do a continuity test on the rectifier to make sure there is continuity in only one direction. I found a post with the instrutions on how to do this so I figured I would give it a try especially based upon how many posts I found with bad rectifiers causing the types of issues I have. The volt meter shows the batteries are being charged and the tach works so I am expecting the rectifier to pass this test. I haven't tried disconnecting the yellow wires while under way yet. The rectifier is almost 2 years old now.

randy, I have not tried replacing the switchbox yet. That point would come once if I decide to start throwing parts at it which I haven't reached yet. And I haven't come across a DBA meter that I could use to test these things. I'd have to do a lot more reading to figure out what I am doing if I were to find such a meter. As far as I can tell the coils are good. I haven't tried running the motor in the dark to look for bad wires. That's not a bad idea.

EDIT: Well, that continuity test didn't tell me anything. This is what I was going by:

"Test continuity or each yellow to the red then reverse your test leads and retest, you should have continuity in only one direction on each yellow to red. Any difference then replace the part."

I pulled one yellow wire off and tested it against both red wires. Nothing had continuity through the rectifier.

 

[CharlieB]

Re: Fluctuating RPM on 96 Mariner 115

"Test continuity or each yellow to the red then reverse your test leads and retest, you should have continuity in only one direction on each yellow to red. Any difference then replace the part."

I pulled one yellow wire off and tested it against both red wires. Nothing had continuity through the rectifier.

Is your meter 'self-ranging' or do you have to turn the selector?

Disconnect the rectifier, touching 12 volts with an ohm meter will instantly blow the internal fuse.

Touch the meter leads together to check for a 'Zero' reading.

 

[capslock118]

Re: Fluctuating RPM on 96 Mariner 115

My issue sounds similar to yours:
http://forums.iboats.com/showthread.php?t=453097

I am in the process of putting the carbs back onto the engine after cleaning and I'll be doing a link and sync afterwards. Oh, I also put out the money to replace all of the internal fuel lines (under the cowl the connects fuel pump to carbs etc) because they were all severely dried and cracked over the ages; I wouldn't be surprised if air was leaking through.

Anyways, point is, I've been told by my mechanic after a brief passthrough with a tester (it looked like a big and fancy ohm meter but maybe it was this dba thing you are talking about?) that my switchboxes at some point need to be replaced because they were not giving consistent readings.

that was during an unrelated test - the fact that you are veering toward a switchbox replacement is interesting, maybe they have more to do with rpm fluctuation than I thought...and not being a mechanic it's not like I would know

 

[pinellas50]

Re: Fluctuating RPM on 96 Mariner 115

I have a standard $12 multi meter. It has a switch selector for different things. One setting is continuity. Touch the leads together and you get a tone to let you know you have continuity.

Caps, I had thought about the switchbox. But you can see earlier in this thread the switchbox doesn't effect flucutuation of rpm. With an engine miss, mabey I guess.

Heck, at this point I don't know if I am chasing two different problems that are showing themselves at the same time.

 

[Maxz695]

Re: Fluctuating RPM on 96 Mariner 115

sounds like a cracked barrel on the thottle cable in the engine or posible in the throttle I,ve seen them crack and then your cable is slipping

 

[Maxz695]

Re: Fluctuating RPM on 96 Mariner 115

look for the obvious and somethign very simple like coil wires getting wet maybe caused by a leak under pressure you guys are shooting in the dark i think soory just a suggestion

 

[pinellas50]

Re: Fluctuating RPM on 96 Mariner 115

I will look at all that stuff. I had tried moving the throttle arm on the motor manually to see if there was any type of slack or play in it that might explain the fluctuation. Everything is rock solid.

I feel like I am looking for a needle in the hay stack in the middle of the night to be honest. These things don't behave at all like car motors I am used to as far as diagnosing problems.

 

[pinellas50]

Re: Fluctuating RPM on 96 Mariner 115

Well, I just found and ordered this:

So I'll have to figure out how to use it correctly once I get it and see what testing with this will tell me.