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1995 25 hp 2 stroke mercury keeps siezing

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redroscoe

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I am new to boating and I just recently purchased a bass boat with a 1995 25 hp 2 stroke mercury outboard. The boat had sat idle for 10 years before I purchased it, After a little work the boat started up and ran fine. I took it on the lake two separate times for a total of 6 hours. The last time I took it out the prop hit a rock and broke rubber sleeve. After a week I put on a new prop and returned to the lake, the motor was seized. I put the mystery oil in each cylinder over night and able to break it free. Started up a little on the rough side and could only keep the engine running by using the idler on motor. I turned off and it seized again. I have ruled the issue is with the lower end. Any ideas? thanks
 
Thanks. I am sorry, it should have said I ruled out the lower end. I already have removed the lower end and that is not the cause of the issue
 
OK I was afraid of that. I never new there was a difference in the oil. If the cylinders are scored, do you know anyone that can have it bored out. I think I have a "metrosil" (sorry not with engine) engine. From what I read it has the ceramic walls and can be bored, I just cannot find where I saw the article. If the cylinders are not scored, is it just a matter replacing the bearings?
 
If you are just running regular 2 stroke oil (like for lawnmowers/chainsaws) that in itself would not cause the pistons to seize. It (normally) would cause lots of blue smoke.

TCW-3 marine oil will completely burn up in a water cooled motor. Regular 2 stroke oil is designed to burn up at a significantly higher temperature - an air cooled engine runs way "hotter" - so if anything you should have an over abundance of oil in the cylinders - "if it's getting there in the first place".
 
Lot of good info thanks. When I first started up there was a lot of blue smoke, I thought it was because it had sat for 10 years. Once it ran for a while there was no blue smoke.
 
II wanted to thank everyone for the help. I did a compression check on both the top and bottom cylinders and both came out at 130 PSI each. I took the exhaust manifold off and took a pic. there seems to be a lot of what I think is carbon from the ring down. Is this considered scored? I have freed up the flywheel and I am able to freely move. I am wondering now that it is freed, if I put in marine oil in the gas, will it run fine or am I screwed?
 
pic did not download here it is photo.jpg
 
ok thanks it was kind of what I had suspected. I was thinking of trying to rebuild this my myself. what does this entail? I think I need to have to rebored?
 
Depending on your mechanical ability a rebuild is not that difficult really - just a step by step tear down, and replace what's busted as you put it back together.

Now, your model being a 1995 puts it right on the hump of can be rebuilt and can't be rebuilt.

If your serial number is 0G202749 or below it has the chrome plated cylinders which can't be bored or honed (if required). If it's 0G202750 or above it has the Mercosil lining and can be bored up to .030" over.

If you have the model that can be done I would first check for an aftermarket source for pistons and rings. Merc does stock them but you are looking at about 300 bucks a cylinder just for a piston and rings. If you crack her open you may as well do the bearings while you are in there (roughly 100 bucks all in) and then a powerhead gasket set ($110) - so you are somewhat north of 800 bucks for parts, plus 50 bucks a hole for a bore/hone (average shop price and not something you want to mess with - especially Mercosil - unless you have done it many times before).

Then you have just fixed the "result" of the "real problem", which was probably a cootie in the carb. So toss in a carb kit (another 75 bucks) and you are coming in at 1000 bucks.

Not trying to scare you away from it, but you can see it quickly adds up and personally I wouldn't spend a grand on this. That's about the going rate for a good used model.

You could strip out what's good, sell it on ebay/craigs list and make almost that in the parts (a working igniton is worth a couple hundred, 2-300 for the lower unit etc).

On the flip side, if you could find the pistons for half the OEM price and come in nearer the $600 mark for the whole thing - you would have a like new motor with a 20 or 30 year lifespan if properly cared for.

Personally, I would grab a Seloc's manual (excellent for doing a rebuild - about 35 bucks) or a factory service manual ($80 bucks but better if you are mechanically familiar with outboards) and tear her down and get a good look.

You can make your decision from there based on how much is toasted. If, at that point, you figure it's cost prohibitive (after pricing parts etc), then at least it's all torn down for selling off the parts :)
 
This motor is far beyond a " band aid fix " as it needs to come apart.----Lucky if it is the style that can be bored out.------A good used tested powerhead is the best option.
 
thanks again for the replies. Could the scoring have been caused because it had sat for 10 years? I am mechanically inclined, but know very little about engines. If the compression is still good and the motor is freed up why would it not run?
 
The cylinders on the motor are machined to within thousandth's of an inch in tolerance. Likewise the pistons and the rings are the same.

The piston is going up and down in that cylinder almost 6000 times a minute when running at full throttle. If those parts are scored up (gouges, burrs, groves) the extra friction where the scoring is causing that piston to twist, albeit minutely, in the direction of the scoring. It will do nothing but get worse until the stress on the pistons connecting rod causes it to break (or causes the rod bearing to let go) and then, at 6000 rpm it will fire that rod through the side of the engine block.

If you are lucky it won't hit anyone on the way by, but in any case the motor will be "beyond repair" at that point.

Now, to answer your questions more specifically -

No, the scoring was not caused simply by sitting for ten years. The scoring would have been result of (in most cases):

Lack of oil to the cylinder - could have been caused by a blockage in the carb, improperly mixed gas, running old/dirty gas OR

Significant carbon build-up - again related to the above reasons.

Secondly, yes, the motor will still run until what I describe at the onset occurs - and if you run it enough in that condition significant other damage WILL occur, and most likely when you are furthest from the launch point..
 
OK thanks for the explanation,I certainly don't want anyone to get hurt. on to the rebuild. Could this much damage have been done in 5 hours of operation?
 
That damage can be done in a manner of minutes.----Likely the reaon it was abandoned 10 years ago too.-----Depending on where you are a used block and used innards are plentifull.
 
in the mean time I did check the rings tonight and they did not seem springs. Then again I did not want to put a lot of pressure on them
 
well I have freed up the flywheel and I did soak both cylinders with sea foam spray over night. I also got some sea foam engine treatment and put some in a gallon of fuel. It seems to run a tad rough, but I think it is normal for a 2 stroke. When I took off the exhaust the gasket now needs to be replaced as well as the one on the cylinders. When I ran the engine I notice a little stream of water coming from the exhaust cover and the power head. I know that is because the gasket needs to be replaced IS that normal that water is in there? any feed back would be great
 
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