Rookie Rebuilding a1989 Mercury 115 4 cyl.

I have worked hard my whole life and rebuilt Tractors, Chainsaws, trucks and lots of other tools. But I decided its time to relax so I bought a boat with a rebuild-able motor and after I rebuild the motor, I am going fishing. My manual is on the way, I am gathering up the special tools, And am reading everything I can about this job and just wanted to say a sincere thanks for all the great info you all share in here. I have had lots of questions answered already by just looking around. Cant seem to come up with anything you haven't covered , But I'm sure you will be hearing from me again. Thanks again.

The factory manual is what you need...a digital camera also helps to take pictures to guide re-assembly...a propane torch and patience is what is needed before applying any excess force to stuck bolts...you might want to start looking for a parts motor...the prices of some of the parts can be excessive...are you sure the motor is rebuildable??? What makes you believe it needs a rebuild????

Yup, Those are some of the questions I had and got answers to by just poking around in here lately. I am expecting my Factory manual any day now. Think I am going to just video the whole thing and I got lots of containers for assemblies and hardware. I also pick up my parts motor tomorrow night which after a little testing will be the one I rebuild. The motor on the boat is an 89 Mariner 115 I looks real clean but has some hours on it and a #4 broken rod. (Per the previous owner). The one I get tomorrow is an 89 Mercury 115. From the reading, and looking, its exactly the same except the colors and decals?
This one looks like a new motor and per the owner, has very low hrs as the boat sat in a pole barn for 11 years before he bought it for the motor. He put it on his boat and the water pump failed and the warning did not function. It started making a ticking noise and has low compression in the #1 cyl.
I am hoping after i get them both in one place and do the autopsy's, that I can get one good motor out of them. (And a spare everthing for the shelf) If not, I should be able to double my investment selling parts.

Yes, the only difference between the 115 Merc and Mariner is the paint and decals like you suspect - everything else is identical so you should have lot's of spare parts.

These are Yamaha designed and built (for Merc) powerheads often referred to as 2+2's. It runs basically as a 2 cylinder at idle (upto about 1800 rpms), then it runs on all 4. The crossover point, both on the way up and back down can seem a little "violent" to those not familiar with them.

So pay particular note to what carbs go where. The lower carbs (for 3 and 4) don't have idle adjustment screws.

Well here is an update. I finally got to it, a couple weekends ago. I did the autopsy's on both motors as I tore them down. Kept every part separately marked, tagged and bagged. Took a ton of pictures and thank goodness I did because I used them a lot on reassembly.
The Mariner not only had a broken rod, it had busted at the rod cap and the pieces blew out the silver-dollar size holes it made in both sides of the crankcase cover @ cyl #4. Surprisingly, the cylinder looks good and the whole engine block measures up good too, but is worthless wihtout the matched crankcase cover right? This engine appears to have had an oil pump failure.
The Mercury was run with a bad water pump and scored #1 and #2 pistons A compression check from the top down said 80,108,112,109. I took the block to my local trusted machine shop and he honed them out and it was within speck when he finished.
I measured and inspected the pistons and luckily 4 of them were good.
It took a couple weeks of cleaning, scraping, measuring, cleaning, and lots of reading and research on this site, but I am almost done. Lots of parts waiting in that time so I rebuilt the carbs, water-pump and fuel-pump. It fired it up last night ((on 25-1 gas)from a clean jug) and so far so good. I just let it Idle fast while I looked for leaks about 5 min. I then did a compression test and got 120,121,120,120.
NO WAY I could have attacked this with-out the help of you out there that have shared your knowledge on these forums. The manual was good to have but not written for a rookie. I like to know the whys, not just the hows, and you all are much better at explaining that than any manual.
Break-in procedures, Gas tank cleaning and flushing, and charging system testing, are my next lessons.
Thanks a ton, I am really enjoying this project.

These are Yamaha designed and built (for Merc) powerheads often referred to as 2+2's

Click to expand...

That is incorrect, only the 4 stoke 4 cylinder are yamaha made