Mariner 25 gummed up plugs

superkrusty71

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HI Guys,

25 Marainer , 1982 , Ser # 695-553601 (Japanese)

Looks like light brown slime on plugs. Runs ok for a couple of minutes then the plugs gum up with this stuff. New and clean fuel, cleaned carby (was spotless), new plugs.

Compression test of 120 PSI on both cylinders.

Motor had sat for 4 years.

Water pump is seems fine and is pumping out the telltale.

I am thinking maybe water leak somewhere. As compression test seems good then I dont think its the head gasket.

Any help very much appreciated.
 

Bosunsmate

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Re: Mariner 25 gummed up plugs

If its being sitting for a while it may of had some moisture in the crankcase and hence the brown oil but that would clear relatively quickly after a few cleans.
Could be a cylinder leak nearer the crankcase meaning it might not show in the compression testing, a leaking exhaust could spray in water too
 

wrench 3

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Re: Mariner 25 gummed up plugs

It might be worth checking the fuel pump diaphram. I don't know about brown, but a ruptured diaphram will definately foul the plugs.
 

superkrusty71

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Re: Mariner 25 gummed up plugs

Could be a cylinder leak nearer the crankcase meaning it might not show in the compression testing, a leaking exhaust could spray in water too

Thanks for the reply Bosunmate, Can/should I take the head off to have a look ? Can i take the head off while its still in chassis ?
 

superkrusty71

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Re: Mariner 25 gummed up plugs

It might be worth checking the fuel pump diaphram. I don't know about brown, but a ruptured diaphram will definately foul the plugs.

Thanks wrench 3 , I am going to have a look at that right now. If it has a hole is it something that is usually visible or a little pinprick ??
 

Bosunsmate

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Re: Mariner 25 gummed up plugs

Thanks for the reply Bosunmate, Can/should I take the head off to have a look ? Can i take the head off while its still in chassis ?


You could take the LU off and put a hose on the inlet pipe. Remove the sparkplugs and see in there if any water is intruding from anywhere. You could also leave the hose running for a while and see if it pools anywhere in the crankcase by tipping it up and seeing with carb removed if any drips out of the inlet reeds.
Posting a picture of the sparks in this stuff would be useful to see
 

CharlieB

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Re: Mariner 25 gummed up plugs

Most likely water mixing with the oil/carbon, making a nasty chocolate and fouling the spark plugs.

A leaking exhaust side cover gasket or crack in the water/exhaust separator plate allowing water to spray back into the exhaust ports into the cyls.

Be very careful in removing the side cover bolts, the little 1/4 - 20 have a nasty habit of siezing and breaking off if you just put a wrench on and trist them. They need to be carefully coaxed out, a small wrench with only a little pressure the a very smalll ball peen hammer and a rapipd tap tap tap tap on the head of each bolt to break loose any corrosion on the threads.

PB Blaster never hurts, when the bolt begins to turn, at all, stop, retighten it PART way, the back out again, repeating turning in, then out, each time getting a little bit more turn(s). Once you think it is going good, STOP, WALK AWAY, and let it cool off. You do NOT want to heat it til it siezes again.

Come back in 30 minutes, another squirt of PB Blaster and continue.

Patience solves many puzzles without damage.
 

superkrusty

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Re: Mariner 25 gummed up plugs

Most likely water mixing with the oil/carbon, making a nasty chocolate and fouling the spark plugs.

A leaking exhaust side cover gasket or crack in the water/exhaust separator plate allowing water to spray back into the exhaust ports into the cyls.

Patience solves many puzzles without damage.

Thanks for the info, I am a mechanic by trade (heavy diesel) so not quite used to these little two strokes. It does look like water mixed in with the fuel/oil. I just wasn't sure how its getting in there if the compression test said all was o.k.

Will pull cover tonight and have a look. I did start to remove but could feel the first bolt was siezing and didn't want to snap it. Will try the softly softly approach

Thanks for taking the time to reply
 

superkrusty

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Re: Mariner 25 gummed up plugs

OK, I tried to take the exhaust cover off gently but the bolts were to seized up. I snapped about 6 of them so this will mean I have to pull the power head off to fix. Managed to get the power head off and found the source of water getting into the cyclinders.

There is a hole the size of a golfball in the exhaust , see picture attached. @#$%#@$%

My question is can this be fixed ? The engine is in really good condition except for this hole.

mariner25e.jpg
 
Last edited:

CharlieB

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Re: Mariner 25 gummed up plugs

See parts here Merc/Mariner 25c

http://www.mercruiserparts.com/Show...ng&inbr=269&bnbr=170&bdesc=DRIVESHAFT+HOUSING

That exhaust manifold comes out of there, it is obsolete and NLA. You 'might' get lucky and find one at one of the marine recyclers, I'd find someone that is very handy welding alum, cut out the side, bend a plate to fit and weld it in using 'soft' filler/rod to reduce weld pull.

Exhaust gasket also NLA, find appropriate material and make it.

The exhaust side cover is usually the culprit for water leaking/spraying back into the exhaust ports and wetting/fouling spark plugs.
Piston, rings, and cyls can be fine, no compression problems, until it begins to rust.

Check the exhaust side cover and separator plate and gaskets for flatness, cracks, holes, and leaks.

Another parts page for the exhaust side. Items 24, 25, 26

http://www.mercruiserparts.com/Show...269&bnbr=60&bdesc=CRANKCASE+AND+CYLINDER+HEAD

If the motor is mostly in decent shape this can be repaired without too much difficulty, even if the exhaust divider plate is totaled one could be cut from hard alum plate IF you know the right guys. Machine shop time could get expensive otherwise. Good project.
 

quicktach

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Re: Mariner 25 gummed up plugs

I'd take it to a good welder and get his opinion. There are some around who do a lot of aluminum work.....
and it's good to know there are also aluminum brazing rods too ...but I'd leave it to a pro.
 

superkrusty

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Re: Mariner 25 gummed up plugs

exhaust-separator.jpgexhaust-separator2.jpgexhaust-manifold.jpgpower-head-exh-cover-off.jpgThanks Charlie.

So the hole in the exhaust manifold would not be allowing water back into the engine ?

I got the exhaust cover off and it doesn't look like there are any holes in the separator plate. Maybe leaking around the gasket but hard to tell. I have attached a couple of pictures for you to have a look at.

Thanks for everyone's advice so far
 

CharlieB

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Re: Mariner 25 gummed up plugs

That definitely is water getting into the top cyl.

Clean and inspect that plate very carefully, make sure the gasket surface and the block are flat, during assembly IF you use anything on the gasket, as thin as possible and left DRY, wet assembly often allows a gasket to 'slip' and tear unseen.

The hole in the exhaust bell will not allow water back into a cyl, it may change exhaust back pressure, maybe a bit louder, possibly a tweek of the idle mixture, as it enlarges it can have an effect on the power curve of the motor, leaning main jet mixture too much.

Easy enough fix while it is apart.
 

superkrusty

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Re: Mariner 25 gummed up plugs

Well I can get the parts I need from a wrecker so thats good. I have 8 broken bolts to deal with now. I have drilled them all , soaked in wd-40 (pb blaster type stuff i guess) , heat , gentle persuasion and an EZ-Out and I cant budge any of them.

Should i drill and try and re - tap or drill and fit a recoil , heli coil type insert ?
 

wrench 3

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Re: Mariner 25 gummed up plugs

It's hard to drill them out in aluminum. Once the drill brakes through the side of the bolt, it wants to walk off into the softer material. If you know someone with a mig welder, hold a nut over the end of the bolt, weld down through the center of the nut and build up the bolt welding it to the nut. The welding does a better job of heating the bolt and you now have something to put a wrench on. Let it cool down before you try to remove it. Get it working back and forth and apply penetrating oil.
Its usually worked good for me. Good luck!
 

CharlieB

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Re: Mariner 25 gummed up plugs

Patience, you get in a hurry and then things go wrong.

Removing broken bolts is a trick, maybe an art, it take T I M E . (Long time)

PB Blaster works well, some swear by other brands, some mix ATF and acetone, they ALL work. If you give them enough time.

And heat. Daily.

Being retired I have time, so when I break one of (it happens to us all) I heat it up smoking hot, then start spraying the PB, using it to cool the part and soak in around the tread as it cools.

If it don't turn, make sure it is well wetted, and leave it, overnight.

Next day repeat the heat and soaking, One day it will turn.

If you get impatient you can drill the bolt out in steps, make SURE you are centered, if the bit breaks out of the bolt that alum is like butter.

Drilled out in steps to the appropriate size for the original tap, you can 'carefully' re-tap the hole, cutting the old threads out and leaving the original thread.

This again take PATIENCE, you cannot muscle the tap and just crank it in, you have to ease it in 1/4 to 1/2 turn at a time, back it out at least as far as you turned it in each time to break and remove all chips.

Slow and steady, and you will have a much better result.

When all else fails, a heli-coil works well, IF, it is installed correctly. They are a great product, but if a mechanic takes his time, he won't need many.
 

superkrusty

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Re: Mariner 25 gummed up plugs

Ok, I have put in helicoils (thread inserts) cut gaskets and re-assembled everything. NOW the motor is locked up !!. After moving the flywheel back and forth a lot I can now move the flywheel around about 4 inches. I pulled the exhaust cover off again to make sur no rages etc were left inside but all ok.
I guess it has been sitting for 2 months while I have been working on it. If it was seized due to rust then I would have though once i got a few inches movment on the flywheel it would start turning over.

Any ideas from anyone would be much appredciated again

I dont want to let the #$%#% thing beat me. Boat motors to me seem like 1 step forward then 10 back

Cheers

Brad
 

oldman570

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Re: Mariner 25 gummed up plugs

If the motor was turning over to get a compression check then something is not good. When doing the heilcoil job did you use new bolts that were short enought so as not to bottom out and push the bottom of one of them causing the cyclender to be pushed in some. Only other things that will cause the trouble is the componets under the flywheel came loose and are hitting the magnets or the bearings have correded and not letting the crank spin freely. JMO
Oldman570
 

CharlieB

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Re: Mariner 25 gummed up plugs

Or the gearcase has rusted and seized.

If you haven't rebuilt the water pump now would be a good time to drop the lower, then see if the motor turns.
 
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