1990 Merc 90 HP Help

[scolba]

Hey Guys,

This is probably going to be a long story, so bear with me! I have a 1990 Fisher SV18GT that originally had a 90HP Merc. These boats actually came w\ a 115 Mariner, but the guy who bought the boat new didn't want a 'Mariner' on his boat, so had them replace it with a 90 Merc....

Anyhoo....When I bought the boat 2 years ago, I made the big mistake of not taking it to a shop before buying it. I did understand it to have been rebuit once, and even have an old connecting rod w\ the bearings looking in pretty sad shape. When I got it out for the first time, it make a banging noise that was seemingly tied to RPM, though probably 2-3x slower than the actual rpm. It actually ran pretty well, other than the noise and carbs needing rebuilt. At one point the boat was running like 45mph on speedo....but the banging was definitely something wrong.

So from there I took it to the shop, and they told me that it had what they call 'piston slap'. They could confirm by digging into the motor, but said it would cost probalbly $300 or more to do that, and that they were confident that it needed a rebuild. So they offered me a rebuild at $1900, a used powerhead for $1700, and a 1989 low hours 75HP Merc for $1400. Being the tightwad that I am, I took the 75HP. They swapped it on and gave me the 90 to take home. And there it sits in my garage, minus a few electronic parts that I scavenged after a fire on the 75 (trim switch on the boat got stuck open and caused the relay to overheat and catch on fire...awesome.), and the stainless prop that I sold already.

So fast forward to now....the 75 barely gets me to 30mph on a good day, and thats no good. The 90 is in my garage, and winter is coming...so that means project time. I'm thinking if I can get the 90 going, that a little over 40mph will keep me satisfied for now.

Now, a friend of mine use to work at a marina on the IL river. He said that they used to have rental boats come back w\ a banging noise similar to what im describing, and that they would just replace the Lower Unit and it would be fine and dandy. So my first question is.....does that thoery have merit? The 75 and 90 have the same lower unit, so in theory, I could swap the LU from the 90 onto the boat now and see if it makes the noise, but is it worth the time??

If not, and I truely do need a rebuild, then I think I'm gonig to take it on myself. I have some experience w\ cars (rockcrawlers, really) and snowmobiles and am mechanically inclined. I was thinking about getting a Seloc manual, but do those contain rebuild info? The other issue I am having is Seloc's split at 1989. One manual is from 65-89, the other is from 90-2000 or somehting. My 90hp motor was on a 1990 boat, but looks identical to the 89 75 hp, so would the 89 seloc be the same??

Then finally, if I do take on the rebuild, is there anything special that I need to keep in mind? Any other advice?

I know thats a lot of info to go through there. Sorry about that!

Thanks a lot for any help you guys might give!

Attached is a pic of the 90 when i first got it....

[scolba]

hmm...just looknig over parts lists, it looks like pistons, rods, etc are the same from a 70-90hp motor in this year....so what else would it take to turn the 75 powerhead into a 90? That might be a little bit easier, eh?

[outboardsanonymous]

Scolba, I can't tell you what the rebuild or changeover would take. If your 90 is ready to go, though, and you want to test your friends theory about the lower unit, I have a 1989 70 HP Lower unit here for sale and if you want to bring your motor down my way, we'll slap it on and try it out. It's up for sale, though, so we'd need to do it before it's gone, but I'm only about 90 minutes south of you, so you're welcome to bring it down. If it works, I guess you'll have found a lower unit, too. outboardsanonymous@gmail

[Faztbullet]

The 70-90 hp engines from 1987 to 93 are 71.1 c.i and then they went to 84.6 c.i. To make your 75 a 90 just install the old 90 carbs as this is the difference. The lower units are the same gear ratio and to check your old one(90) just pull the bearing carrier and inspect gears and bearing, that the easy way..