Old Mercury 900 90hp Outboard - Assessing and starting it for the first time in 10yrs

[MichaelV]

Hey guys,

A buddy and I just purchased a 16' 1968 Larson Lapline for a few hundred bucks and she came with a Mercury 900 90hp outboard. The elderly man we bought it from was convinced he could get it running if we let him try for a day even though he hasn't started it or even had it on the water since he traded his neighbor for it over 10 years ago.

Anyways, my friend and I are going to try and get it going and were wondering if anyone could give us any advice. We are both decently mechanically apt and have rebuilt vehicles before but this will be our first boat project. Is this outboard a good platform to clean up and get running? We don't know what year it is but I think I read that it is a 78-79? The serial number is 4846153.

We look forward to the project and conversing with you guys since we have been lurkers on these forums for a while. Thanks in advance for your help!

Michael and Sean

 

[emckelvy]

Re: Old Mercury 900 90hp Outboard - Assessing and starting it for the first time in 1

Here's a good article to read on resurrecting your 90hp Tower of Power:

http://forums.iboats.com/engine-fre...kening-sleeping-outboard-boatbuoy-158086.html

Pretty good advice. At minimum you should take out the spark plugs, spray some light lubricant in each hole and turn the flywheel over by hand. Stick the plugs back in the spark plug boots, ground each plug. With a fresh battery connected, turn the ign swith to "ON" and pull the flywheel over with a rope wrapped around. You don't need to crank the engine with a starter to check spark, if you've got a hot spark you'll get it even with the rope-pull. Check all the internal wiring for cracks/breaks/deterioration. The internal harness is notorious for dry-rotting. If you get no spark, there are troubleshooting tips for that.

If she's got good spark, put 'er back together, pump up with fuel, apply water and crank 'er up. This is the minimum you should do before starting. Likely you'll note other issues as soon as you attempt to start.

Note that even if she fires up, you should plan on an impeller replacement before taking out on the water. An old impeller will have taken a "set" and won't pump well. Also, the old rubber could break off and then you'll get pieces of rubber stuck in your cooling system. Not Cool!

I'd also recommend replacing the fuel pump diaphragm/gaskets. They will have dried out and may not pump well. More-or-less routine maintenance.

Now, if you're really lucky, the carbs won't "puke" gas all over, or be all plugged-up inside. If they are, you'll need to dig deeper as per the "awakening" instructions. Normally you won't need a "rebuld" kit, only a "carb packing" kit for each carb (relatively inexpensive).

Drain/refill the gearcase oil, use "Hi-Vis" marine-grade gear oil. Wal-Mart usually carries some in their small boating section.

If the motor seems to run reasonably well, it's probably safe to take 'er out for a bit of Sea Trials, but don't stray too far from the dock or beach.

Warm up fully and check for water pumping/good idling/shifting/etc.

If the in-water test works out, time for a full tune-up including link/sync (of which you'll find instructions in the repair archive), distributor service (clean/inspect cap, etc), any other odds-n-ends.

If the motor was layed-up properly, it should do well. You just never know until you get into 'em and see what you've got.

Just remember, the powerhead is probably worth more than what you paid for the whole rig, if you burn it up and were to have to get a replacement! Properly fed and cared-for, your Tower of Power should give you many years of good service.

Last thought: Probably a good idea to do a compression check; if you've got a bum cylinder, no sense in putting a lot of work into it for naught. Do the check with all the spark plugs removed. You can "jump" the solenoid with the key off, by applying +12V to the yellow wire on the starter solenoid. Ideally all compression readings will be within 5 psi of each other, but if they're a bit more uneven, you can still run the motor and decarbonize it afterwards with Seafoam or OMC Engine Tuner, and sometimes that'll help if the rings are carboned-up.

If the compression readings are, for example, 30 psi or more off from each other, not so good, and if they don't improve after a decarbonizing treatment, time for a teardown.

HTH & let us know what you find..........ed

p.s. lots of replacement parts here at iboats.

Check out these diagrams for your motor, elsewhere:

http://www.marine engine.com/parts/mercury-outboard-parts/90-6-cyl (note: remove space between "marine and engine", this site filters competitor's url's)

Mercury Outboard Wiring diagrams -- Mastertech Marin

 

[narvlebenoit]

Re: Old Mercury 900 90hp Outboard - Assessing and starting it for the first time in 1

like Ed. says they are good motors I love the sound of them towers of power.

 

[MichaelV]

Re: Old Mercury 900 90hp Outboard - Assessing and starting it for the first time in 1

Thanks for such a thorough and helpful response emckelvy! My buddy and I have taken the Tower of Power off the boat and threw her up on a stand we made the other day.

You are definitely correct on the dry rotted internal wiring. Most of it needs to be replaced. We read a few other threads of some people highly recommending buying a new wiring harness completely but at almost $160, almost more than what we paid for everything, we decided to start replacing each wire ourselves. With the wires already connected correctly (we hope) and some organization, it doesn't look to be too bad to replace them all and I have started doing so.

We took your advice and bought several things you recommended. We purchased quite a bit off this site like carb rebuild kits, new floats for the carbs, a water pump kit, as well as a replacement starter gear since ours was missing a tooth which I actually found as I started to replace some of the dry-rotted wires.

We also removed all the spark plugs (ordered new ones) and threw some mystery oil in each cylinder. After a few hours, we tried turning the flywheel and it moved relatively easy. We turned it around several times and also tried a bit with the spark plugs in and as far as we can tell, at least a couple of the cylinders have compression. We will do the compression test when we can.

The other day, we threw on a battery and a trolling motor and took the old 1968 All American 16' Larson out to a small local lake. (It took us quite a bit of research but we finally figured out the year, make and model of our boat! Seems like quite a few people restore these things so we are happy about that). Anyways, we were able to drink several beers and tried to fish for several hours without sinking! Even after pulling her out, not a drop came out so that also made us happy since we had no idea about the seaworthiness of our new boat. We are now debating on whether we want to pull up all the floorboards and replace them or just repair two small sections which are much worse than the rest.

All in all, we are excited about our new project and the help we have received on here. Thanks again!

Here are a bunch of pictures of the project!















Here is a link to the full album of pictures so far. Any advice or comments are greatly appreciated!
http://s276.photobucket.com/user/mik3v/library/Miss%20Daisy%20-%201968%20Larson%20with%20a%20Mercury%20900
Slideshow: http://s276.photobucket.com/user/mik3v/slideshow/Miss%20Daisy%20-%201968%20Larson%20with%20a%20Mercury%20900

 

[Texasmark]

Re: Old Mercury 900 90hp Outboard - Assessing and starting it for the first time in 1

While you have her down to the "cloth", check carefully where the deck (floor) is glassed to the hull. On some older boats with stringer problems, the hull will flex and the deck won't and there will be cracks in the glass across the interface. I see glass there in your pictures. Wonder if that's OEM or added later for the reason specified?

The reason I say this is I was looking to buy a boat that looked like that and not sure as to the brand now, been 40 + years ago. It had a white 75 hp Johnson V4 with the old style bullet nosed lower unit, small prop and no thru hub exhaust to try and date it. On the test run what I told you happened to me. Every time I hit a wave the floor would move up from the hull at the interface. Obviously I didn't buy it.

The problem with that is that water can enter the area under the deck which, years ago, was substituted for foam flotation; a "sealed" air box if you will. Stringers were not protected as this was supposed to be a dry area and over the years rotted. IF this is your case, aka you see cracks and or splitting, etc., you would do yourself a favor by biting the bullet now and removing the deck and replace the stringers and probably the deck too as the underside of it was probably not protected either and you could have soft spots from rot. It won't cost that much, time consuming yes, but when finished you will have a nice solid boat and will thank yourself for going the extra mile. The work will be in the past and just fun and enjoyment to be had.

Mark