I have a 2000 model year two stroke Merc 90. It is the 3 cylinder variant. The boat sat for a few months with non ethanol treated gas. I started it up this past weekend on the hose. It was more difficult than usual but it did start, idle, and sound fine.... after some coaxing. My question is, should I use seafoam fogger on the air intake of my outboard? The boat seems to run fine so I'm not trying to resolve an issue. I have a can leftover from use on lawn mowers and other small stuff. Figured it might do some good.

will/Could running seafoam spray through the air intake make the outboard run better? Can I screw something up by doing it?

I wouldn't.
You should get in the habit of starting it up once a month and letting it run for a few minutes.

Clean the tank , replace fuel line or at least flush it.  Rebuild the carbs.

Doing it half assed isn't worth it.


Run marine sta-bil and try to get sport fuel without alcohol.

Spraying "stuff" through the carbs will only clean the carb throat and reeds and some carbon from the piston tops, it won't clean any jets or passageways. Best thing to do is remove the drain plugs on the float bowls and see what they look like. If they are clean brass, then the carb internals are most likely fine and running some Sea Foam or whatever in a tank or two of gas can't hurt.

If the drain plug has crusty green crap on it then I'd tear the carbs apart and clean them out. Mercury's Power Tune spray works great for cleaning brass jets and emulsion tubes.
ETA - I agree with akethan the best thing would be to tear them down and rebuild along with the fuel pump. But then again, we both do this for a living.

Thanks so much for the info. Is it easy to find the drain plug you are talking about? Also the engine will start and run on the hose. It was tough to start a few days ago after sitting for five months but that's not surprising. Once started it seemed to run on the hose just fine. I did use stabilized non-ethanol fuel so hopefully that minimized any of the nasty side affects of stale gas. I'd prefer to not tear it apart unless it is necessary. That's something I would have to pay someone to do (unless it's pretty straight forward).

By the way, is there a service manual or chiltons type book for this outboard that you would recommend?

Thanks for all the input guys. I definitely slipped up by not starting the boat on the hose every month or so.

The drain plugs are on the side of the carburetor's float bowl. It is a small brass screw. Place some paper towels under the screw to catch any fuel.

The carbs are pretty straight forward if you need to take them apart and clean them. There aren't any complicated "gizmos" inside them. Just a float, jet and the emulsion tube.

Make sure you clean every single passage. Remove the idle mixture screw on the side as well as the steel plate on the top so you can clean the passages.

Count the number of turns required to lightly seat the idle mixture screws all the way in. This will give you a reference point for the adjustment.

I've cleaned literally hundreds of these type carbs over the years. IM me the serial number and I can IM you the carb rebuild procedure as well as the synchronization procedure.

You will need to re-synchronize them after removal.

ETA - Part #24 in this diagram is the drain screw. The main jet is part #26. Part #12 is the steel plate you need to remove to clean all the little passageways.

You don't have to remove the throttle plate and shaft to clean, just the float bowl, float with needle, main jet, idle mixture screw and the steel top plate.

Make sure the float height is correct when re-assembling.

If you are careful, you can re-use the gaskets if necessary although it's best to use new ones.

You'll need a timing light to verify/adjust timing.

KB7DX and others, thanks a lot for the helpful information. I got the boat out this past weekend for a few hours. Filled the remainder of the tank with fresh rec fuel (about a third of the tank) then ran it. Filled it again with another third of a tank of new rec fuel on the way in. Then ran it on the hose for 5 to 10 minutes in the driveway before putting it away. It ran fine so I don't think doing anything else is necessary. Not going to mess with the Sea Foam fogger. Running it really seemed to help.

Question, how can I find out what service manual I should get for this outboard? I'd like to be able to do little things like change the impeller, lower unit oil, etc. Just routine maintenance things. The boat would still go to a pro for the more involved matters. Oh also, how frequently should the spark plugs be changed out?

Mercury likes to go by serial number for reference so look up the manual by the serial number so you get the right one. I recommend the factory manual over the Seloc.

You'll pay more for the factory manual, but well worth it.

I can't recall the interval for plug changes off the top, but we changed our rentals out about twice a season or at about 400 hours.

If you post up the serial number I can look up the proper manual for you at work tomorrow.

ETA - Don't be afraid of that engine as far as working on it. They are dead simple and easy to diagnose and repair with the factory manual and some basic tools.

Gearcase repair is more involved and requires special tools, but the powerhead itself is simple.

Great information. Thank you. The SN on my engine is 0T018296. I'd really like to get to the point where I can do basic maintenance myself. My marine mechanic said the model of outboard is a good so hopefully it's got a lot of life left.

While looking for the SN of the outboard I notice a part on the engine that is rusted pretty bad. There is a black piece of metal that attaches on one side to the metal arm that steers and on the other end it goes into the outboard. The part that goes into the outboard is rusted. Can you tell the part that I am talking about by the description? Not sure if I should post or send a pic.... I don't know if this is a major issue or something that isn't major now but will be a real issue if not taken care of relatively soon.

Sorry for all the questions and thanks so much for the help.

Went ahead and uploaded a photo. This is the side of the outboard that is on your left as you are standing behind the outboard. The part seems solid but is pretty rusty with paint flaking off.

That part is the steering arm. It's a beefy forged steel piece and that little bit of rust won't hurt it. Keep it greased well like I see in the pic and it will be fine.

To replace it, you have to tear the power head off and dismount the upper midsection mounts. It's like they suspend the arm from the factory ceiling and build the motor around it.

I'll post up the manual number tomorrow at work. I looked at this thread at work today but you hadn't posted yet.

Glad I can help and post up or IM me if you need any more help.

That's great news. What a relief! Since posting this I read about how involved and expensive it is to replace that part. Seems like some people were saying to use a wire wheel/rust converter on the part (like por15) but maybe they were talking about the other side where the steering arm attaches (which is more accessible). Others said to coat the part with grease. Is there anything I can do to save the part and prevent further rust, to the extent that is possible? Thanks again much appreciated!!

You can use some rust converter on it if you wish. Wire brush it and apply the rust converter stuff, then keep it well greased.

The new manual part number is 90-8m0110566. The old part number for the manual is 90-830234r04. Search the part number online and see if there's a used one on e-bay or the like.

Just got the manual on eBay for $35. Without doing major disassembly, is there a way to expose more of the part to be able to clean it more effectively? The rust is occurring where the part connects to the outboard itself. It is kind of tucked up underneath and hard to get to.

Very much appreciate your taking time to answer my dumb questions!

The "chaps" or pants (lower cowlings) as we like to call them, are easily removed to expose more of the sides of the outboard. Just remove the weather stripping, take out the 8mm screws around the "edges" , a 10mm nut on the front latch, unplug the trim switch, remove pee tube and separate like a clam shell.

There are no stupid questions, just stupid answers.

ETA - I like to use a 1/4 inch drive 8mm swivel socket and long extension to get at some of the 8mm screws that are hard to get to. Makes life 100% easier.

Getting that lower cowling off doesn't sound too difficult. I think it would be well worth protecting that steering arm however I can within reason. Thanks for all the great information. I really appreciate your input!

Looked at a 2008 model today which uses the same cowling and you do have to remove the control cables and wires/fuel line to get the chaps off.

It's not difficult, just make sure the cables are adjusted properly when re-installing. If you are careful not to rotate the cable "barrels" and it shifts good now,

you can just re-install.

The service manual has a correction in it regarding intake and reed block torque values. The manual lists something like 60ft pounds torque where the correction says 110 INCH pounds. Major difference!

Just so you are aware.

ETA - Post up a pic of the entire side of the outboard please. The online parts manuals for your serial number don't match what little bit of the cowling you posted.

Just noticed this. Hopefully this pic is good enough to see what you need to see. Thanks for the correction on the inch lbs! That could cause a big problem. I will need to check out how difficult it will be to install reinstall the cables. I don't want to cause any problems! Thanks again.

Ok, it's the same type as the 2008 models. It will split open like a clam shell.

As long as you don't rotate the plastic cable "barrels" you won't need to adjust anything. Some White-Out on the barrels as witness marks will help.

That's great. Thanks again for all the help!