whats best choice, rebuild buy another used motor?

[udoittwo]

Hello,
I have an 1986 - 75hp Mariner that I have owned sence 1986. I only use my boat for fishing and I am in an area that has mostly electric motors only, so I've rarely used the motor but when I did, it allways started and ran great. Last year it wouldn't start. A marina said the compression was too low in the top cyl. I checked and it is. I have rebuilt motors for my old cars so I am mechanically inclined. I am not a person with a lot of money to spend. My question is, what is the best approach for me? Can it be rebuilt, if so, is it cheaper to rebuild or take a chance on a used motor?
Thanks for any advice,
Karl.

 

[carholme]

Re: whats best choice, rebuild buy another used motor?

What is the s/n of your engine?

What is the compression in all the cylinders?

Easiest thing to do at the moment is get an idea of the damage and to get a look at the rings and any scuffing on the piston. Remove covers, item 30 in the following link and see what you have. May just be stuck rings and with a screwdriver, you can test them for freedom of movement.

Rebuild will have to be determined by what you find. If you don't get any results with the covers off, you may have to split the block to see the condition of the cylinders, pistons etc. Won't cost you anything to tear it apart and may be very reasonable to repair. Possibly better than buying another persons problem.

Gerry

 

[MH Hawker]

Re: whats best choice, rebuild buy another used motor?

Personally I would rebuild. I am in the same boat on money and i really enjoy doing things my self. I have rebuilt 3 over the years and its not especially hard to do. First thing I would do is pull the cylinder head cover and inspect the cylinders then you can see if it needs to be bored out or just honed out. Boring isnt all that expensive last time i had one done it was 25.00 a cylinder. and once opened you can see, then price parts and availability, it all sorts out over time. Thats new pistons and rings, bearings and seals. You may be able to get a rebuild kit. then price a new motor. That is often enough to decide.

 

[racerone]

Re: whats best choice, rebuild buy another used motor?

There is no cylinder head that can be removed on this motor.-----It will be cheaper to rebuild what you have in most cases.----No point possibly buying a problem motor from someone else.

 

[hotrod53]

Re: whats best choice, rebuild buy another used motor?

I have a '77 Merc 500 and it has no cylinder head either, all pistons and the crank come out the bottom of the block all at once. I was told recently that many Merc dealers won't rebuild my motor for that reason. There is a local guy who rebuilds them for the Merc dealer, he's a real piece of work. He takes his good old time doing one of these because no one else will touch it... which I'm sure translates to $$$ too. My motor was rebuilt many years ago and as of late still has good compression. In my case, I'll buy a replacement before I'd rebuild this one... of course mine is a few years older than yours

 

[jbjennings]

Re: whats best choice, rebuild buy another used motor?

There's nothing wrong with buying a used motor if you do the basic checkups. Sure, they usually need a little work but if you check spark, compression, and lower unit oil after running it for a while on muffs or on the lake, and quiz the seller about whether it holds gear or not. Sometimes you can get a great motor that only has a minor problem that is much more cheaply fixed than rebuilding your motor. I definitely would follow the advice given and pull the exhaust covers and just see what's wrong with it and get some idea about what all it needs. The most important thing is finding the exact cause of the bad cylinder so it is fixed when you rebuild the block. I personally would leave all good cylinders alone and not even re-ring if their compression is nice, and no scoring is evident, and only repair the bad cylinder just to keep from having so money in an old motor. I would not be at all scared to rebuild it, but ask lots of questions and get an OEM manual.
Good luck,
JBJ

 

[udoittwo]

Re: whats best choice, rebuild buy another used motor?

Honestly, I am a little leary of "good used motors". I have bought a few used car motors over the past 45 years. I am probably 1 good one out of 4. I'm one of those gullible people. I am allways way too honest and I trust people to be the same.
I am not really concerned about rebuilding the motor itself but all the externals bother me. Everything appears to be nice, pliable wires, ects but there's some much compared to a car. A car you can undo everything and lay it off to the side. I guess I will take a lot of pictures, lable everything as I take it off and go for it.
As I recall, the compression was 155-160-160 in the lower 3 cyls and below 100 in the top one. I will take a look at the cyl as best I can. It allways started and ran great but it allways smoked at start up from day one. That would clear up in a few seconds.
If I remember it right, I believe someone suggested to check for a "stuck ring". Not really sure what that means? I don't know how these rings work. Unless they are different than automotive rings. Did I misread that, are there reed valves in these? Did you say stuck reed? If something could get stuck, that might make sence as it did run good and I sprayed the foam lube in the cyls as soon as I used it last. It just sat for a year but I believe it was well lubed in the cyls.
Oh well, sorry I tend to ramble and thanks for all your suggestions.

 

[udoittwo]

Re: whats best choice, rebuild buy another used motor?

IF I decide to rebuild, are there any special tools needed? If I tear it down on the boat, what will the block with pistons weigh? It is an 1986 Mariner 75hp 4 cly?
I just ordered a shop manual for it. I don't even know if its aluminum or cast iron? Can it simply be lifted or will I need a cherry picker?
I've got a local machine shop that has done all the machine work on my car motors for the past 40 years and I trust them, so they should be able to clean this motor up, if needed.
Thanks again,
Karl.

 

[Scott Danforth]

Re: whats best choice, rebuild buy another used motor?

if it is stuck rings, a decarb could work wonders.

I would rebuild it if it was me.

 

[racerone]

Re: whats best choice, rebuild buy another used motor?

There are internal reed valves on this motor.-----Note ---Reed valves have nothing to do with compression in the cylinder.----You can remove the top bypass cover to inspect piston and rings.----Just a few bolts and a gasket on the starboard side of the motor.

 

[gbrowne9]

Re: whats best choice, rebuild buy another used motor?

i,m with scott danforth do a major decarb and recheck compression then go from there.

 

[udoittwo]

Re: whats best choice, rebuild buy another used motor?

Sorry, but I am totally ignorant. I don't understand what you mean when you say "stuck rings". I only know automotive engines. The rings set in a groove in the piston. Aren't these motors similar? In a motor that I understand, if the ring were stuck, to me, that would mean it was frozen to the cyl wall and the motor could not turn over. The motor turns over, so where, what, or how is the ring sruck?
Thanks for any help as I am really lost here,
Karl.

 

[Chris1956]

Re: whats best choice, rebuild buy another used motor?

Gee, What is the compression on all cylinders? If the low cylinder has any compression at all, you might just run it as is, as you use motor so sparingly.

Usually cylinders with bad rings or cylinders have zero compression.

 

[racerone]

Re: whats best choice, rebuild buy another used motor?

When a piston scores some of the aluminum is dragged over the piston ring.-------------Now the ring is jammed in the ring groove.-----------The ring is no longer free to move and contact the cylinder.--------It will not seal compression.---------The ring is then said to be " stuck "

 

[carholme]

Re: whats best choice, rebuild buy another used motor?

Karl;

The rings are springy and designed to hold outward pressure against the cylinder walls but when carbon builds up, it can freeze the rings to the point that they do not have the ability to spring outward against the cylinder wall. This is why I said in post #2, to remove the covers and using a screwdriver, see if the rings are free or not. Others have said to do a decarb treatment which hopefully will release the carbon and free up the rings. But by removing the covers, you also get a chance to view the condition of the pistons for scuffing which may indicate a transfer of piston (aluminum) material to the cylinder walls.

You still have not provided the s/n of your engine but if you are sure it is a 1986, here is the parts list for an '86:

75 H.P. - Mercury, Mercruiser, Force, etc. Parts Lookup

The covers to remove are item 30 under END CAPS, MANIFOLD AND EXHAUST COVER

Gerry