possible overhaul time 3cyl merc

[les morgan]

Motor--1987 merc 90hp 3cyl. serial #b153941...
On Saturday I was idling along after running around for about 2 hrs and all
of a sudden -thunk- engine quits. I dont mean slows down to a stop I mean
complete stop.. When I tried to start -thunk- no turn of motor. I was towed
in and pulled the plugs, cylinders look clean and cannot feel any obstruction
at top of pistons. But when you turn the flywheel you can go about 95%
of a full turn in either direction and it just stops. I also pulled the lower unit
and no difference.. Is there anything external that might cause this, like
under flywheel, or is it diffently time to open the crankcase... Also there was
no rattle or knock before it shut down.
Now if it is time to open up the crankcase and rebulid the powerhead what
manual would be best. I have the seloc manual but it does not show enough
detail. I have found some manuals on CD (on ebay) but no original Mercury
service manuals..
I have seen on the boat restoration forum people use a lot of pictures to
show their work. Would there be any interest on seeing a motor rebuild with
photo documentation?
Any help and advice on the best way to approach this job or hints I would
be extremly grateful..

Les Morgan

 

[Laddies]

Re: possible overhaul time 3cyl merc

It sounds like you had a rod go out with the plugs remove check to see if the pistons move without any slack, if they seem OK check to see if a flywhool magnet came loose the can act the same way.

 

[mthieme]

Re: possible overhaul time 3cyl merc

I was going to guess a rod cap too.
Although not always, I would have expected some noise prior to the thunk - a sharp rapping...even if something came loose under the flywheel - a screeching perhaps.

 

[les morgan]

Re: possible overhaul time 3cyl merc

Using a phillips head screwdriver I felt all three pistons seem to go full revolution, except it stops when #1 (top) piston gets all the way or almost
all the way to the top. If a rod cap came off would the pistons still go
up and down???

 

[mthieme]

Re: possible overhaul time 3cyl merc

If the cap "came off" clean, one piston would top out and stay there.
Typically, they break with some of the cap still bolted flopping in the breeze and lodge or catch something preventing a rotation.

Can you hear anything when it stops?

 

[les morgan]

Re: possible overhaul time 3cyl merc

I am not at home right now. But when it stops it is just a dead stop. (either
direction) I will listen more closely when I get home this afternoon. Maybe
I will be lucky and find something under the flywheel. I cannot see an internal
problem not making any kind of tick or nock before it happened....

 

[les morgan]

Re: possible overhaul time 3cyl merc

Got home and checked piston movement again. Lower two cylinders seem
to make a complete cycle. Top of rotation and then back down. Upper
cylinder does not seem to come to top of rotation when it comes to a complete stop when turning the flywheel either clockwise or counterclockwise. Next step would seem to be pulling the flywheel. Are there any options to buying the special flywheel puller tool? On my 50hp merc
I could use a 3 prong pulley puller, but on the 90hp there are no spots to
bolt down a regular pulley puller.....

 

[Laddies]

Re: possible overhaul time 3cyl merc

You need one of these. http://cgi.ebay.com/ebaymotors/Merc...9164544QQptZBoatQ5fPartsQ5fAccessoriesQ5fGear

 

[les morgan]

Re: possible overhaul time 3cyl merc

Finally received the flywheel puller I ordered of the internet, pulled the
flywheel last night and found no obstruction under it. So whatever is stopping the motor from going completely around has to be internal.....
I will be pulling the powerhead this weekend but I have a few questions.
From looking at an engine breakdown picture it appears that the whole
carburator setup can be removed in one piece with the removal of the induction manifold-- is this correct? Will I be able to see into the lower
part of the crankcase when this is removed? I am still puzzled on what would stop the crank from turning when there were no rattles or knocks before
motor stopped.
Any advise on what to look out for or any hints on the dismantling of this
engine would be greatly appreciated.

Les

 

[les morgan]

Re: possible overhaul time 3cyl merc

problem found- opened up the powerhead and found this

Bought this motor about 4 mos ago. The person I bought it from said it was
tuned up by bass pro shops. From what I can tell the top cylinder (where this piston came from was running lean. There was about a teaspoon of oil
in the lower cylinder, about 1/2 a teaspoon in the middle cylinder and the
upper cylinder looked dry...
Good news is that the cylinder wall has no scoring at all. I have a spare
piston and rod but the merc dealer says that rings only come in a complete
set for a six cylinder (12 rings) where my motor is only a three cylinder.
Does anyone know where I can get a set of rings for the one cylinder only.

Thanks les

 

[Starppy]

Re: possible overhaul time 3cyl merc

Try the usual suspects like http://www.dougrussell.com or http://www.boats.net. Looks like boats.net has them in a set of 6 (2/piston)Since you have it apart, why not replace all six?

 

[les morgan]

Re: possible overhaul time 3cyl merc

Ok, I have new rings comming for the overhaul. The seloc manual says to
use a gasoline soluable grease on the main and rod bearings when installing.
What kind of grease would you use? I am the service manager for a chevy
dealership and one of my mechanics said I could use vasoline. Is this a good
way to go?

Les

 

[saumon]

Re: possible overhaul time 3cyl merc

Yes, vasoline is ok. And while putting new rings in, you should also deglaze with a ball hone (Flex-hone) to remove the mirror finish and bring back a cross-hatch pattern that will retain oil.

Never, ever, use an automotive brake cylinder hone for this purpose. It will eat way too much material and leave sharp edges on exhaust ports.

 

[les morgan]

Re: possible overhaul time 3cyl merc

thanks saumon I was hoping the vasoline would work. And I had already talked to my tech about using a ball hone on the cylinders, you just confirmed
my thoughts.