I'm more familar with OMC motors but I was working on my first Merc and saw this thread so I thought I'd chime in cause I just changed my plugs in the same year motor today. 84 I was told. SN is 6480845. Does that make sense? Its hard to read. I also used the NGK plug.
I just bought this wooden boat with this 84 Merc 115 and it runs great. I just want to try and keep it running so here I am. Some of you Merc guys have seen me around here and there so just wanted to give you fair warning I might be lurking with some questions in the future.
I'm not good at posting pics yet and the close up of the old plugs is blurry but you get the idea.
Anyway hopefully it'll be a good runner for me if I treat her right.
Good morning Jill, and welcome to FG. The good folks around here are more than happy to help you out with whatever you need to ask, so don't be afraid to do so. I've only been around here for a couple of years, but there are plenty who have spent their lifetime at this.
The outer two plugs in you pic look normal, is that carbon or oil deposits in the center plug? (Carbon would be black sooty but dry, oil would be wetter looking).
No its a Lyman look alike built in Hong Kong in 1958. I've always wanted a wooden boat and this baby fell into my hands for $2500. All Mahogany, 19 1/2 foot. Only had it out twice, mostly used in salt..
The middle plugs looked worse than the upper and lower. Its hard to tell. The threads are oily of course. It looks like carbon on the wet side. It had some corrosion like on batteries on the trim solenoids under the cover and I cleaned them up but only could get 2 of nuts off. The other 2 seem seized on the thread and was turning too as I was trying to get the top nut off. I was using a wrench to hold the back nut steady but thread kept turning. Is it OK to let it pull on out of the solenoid or does that mess up the insides? What kind of grease can I put on them to hinder future corrosion? A few grounds look the same too.
Looks like you got the hang of it posting pics, very good. You definitely don't want to keep turning if the threads (post) is turning, it WILL screw up the insides of the solenoids! You can try penetrating oil like PB Blaster, WD40, or something similar. It may take repeated applications over a period of time, say every day for a week, before they eventually break loose. Worse case scenario i have used a mini ziz wheel like a dremel tool cut off blade to split the nut, just be careful not to damage the threads or the connector on the cable/wire.
When you get ready to go back together with it, you can use dielectric grease (messy) or liquid electrical tape in a can. It has a brush in the cap, and you paint it on like fingernail polish. (NOT a reference because you're female, just the first thing that came to mind. )