mercury 90 hp 2 stroke inboard rebuild

Hi guys, I recently got a free(!) jet boat from a local junk yard. Everything is intact, steering, impeller, etc. However, the motor seems seized.

Lately I've been looking for a cheap or free motor to rebuild and learn on, so it seems this will be the one! I am very excited.

I've done work on car engines and some inboard 4 stroke boat engines before, but have never rebuilt a full engine before, and I'm afraid this is way past my skill level, so I'm turning to you guys for some advice.

I have pulled off the flywheel and stator already: I tried pulling the exhaust, but several of the bolt heads snapped. I know I might have to retap them, and I'm ready for that.

My first question is, what should I do first? Should I unbolt the mounting brackets and pull the engine out of its compartment? I have a room in the house that is empty that I can use for a work room. Should I pull it out of the boat and bring it inside?

I can take pics if you guys need. I am VERY VERY excited for this opportunity to learn! And it's even better since the motor doesn't run; no matter what happens, I can't screw it up worse than it already is!

So what should I do first? Unbolt the mounts and pull it out?

Thank you in advance for any help. I can't wait to get my hands dirty

Re: mercury 90 hp 2 stroke inboard rebuild

Absolutely, pull it out! It isn't that hard and you will need it out anyway. One thing to keep in mind though, that isn't a mercury, nor an inboard. It is actually a force outboard engine with a mercury sticker on it. Being a force, parts may or may not be available, depending on what you need.

Re: mercury 90 hp 2 stroke inboard rebuild

smokingcrater said:

Absolutely, pull it out! It isn't that hard and you will need it out anyway. One thing to keep in mind though, that isn't a mercury, nor an inboard. It is actually a force outboard engine with a mercury sticker on it. Being a force, parts may or may not be available, depending on what you need.

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Haha sweeet. That makes sense anyways, as it's been impossible for me to find the manual by looking up mercury 90 2 stroke inboard. Any idea what I should look for? "force outboard 2 stroke manual"? It is inboard, but it does look exactly like this one outboard 2 stroke I saw with the cover removed, so it makes sense that it would be an outboard engine just put inboard.


Well, how hard do you reckon it'd be to have parts machined for this little guy? I mean it just seems like a huge lawnmower engine, doesn't seem to have too many parts. Also, it seems to be solid and in real good shape, apart from the dry rot on the exhaust housing bolts. Even the flywheel shear key was in real good shape. There's some gunk in the carb and all, but I think that's just because it was a premix? I know this isn't gonna be something quick, but I'm real excited to have the chance to take it apart and put it back together myself and learn.

I'm gonna start the pulling out today, now that you've confirmed it... I'm super excited

And thank you

Re: mercury 90 hp 2 stroke inboard rebuild

If you dig around some you should be able to find the pdf of the service manual. They also sell them on ebay for relatively cheap, under $10.

I'm far from a force expert, so can't answer how easy it is to get parts, but... I would have to imagine that relatively common things that might be wrong in a seized engine like bearings/pistons are still somewhat attainable.

Premix... Those weren't pre-mix from the factory, they are oil injected. Someone might have converted it over after the fact.

Lastly, when you pull the engine out, make sure you are supporting the pump from the bottom with a jack also. At least with the larger SJ's, the pump can fall out when you unbolt it, so I don't know if the smaller SJ's are like that also.

And one comment on the 90/95hp SJ's. They weren't exactly overpowered. The Sj120 was a MUCH better engine and performed substantially better, as well as having more Mercury DNA in its blood. If you could find a totaled boat with a decent shape 120 in it, might be worth considering.