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Old 12 December 2013, 05:21   #1
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Mariner Cylinder Problems

I have an old (circa 1988) Mariner 2 stroke 75hp clamshell and am having problems with an overheat. I've been all through the cooling and warning systems (see my previous thread here) and come to the conclusion that it is a genuine overheat problem, having replaced all of the main cooling parts, checked flows, checked the alarm sensor etc. and opened up the engine to check cooling paths are clear. When I opened up the exhaust cover plate, however, I found deposits on the middle cylinder piston wall where it is exposed to the exhaust manifold.



There is a dark patch on the lower piston ring, but no significant deposits between rings, which made me think it wasn't blowing past the rings.



Anybody know what is likely to cause this sort of thing? I am sure I read somewhere that if it was running lean on a cylinder, then that could lead to increased exhaust temperatures, which would then cause the overheat and oil to burn on the cylinder. Can anybody shed any light on this?

Unfortunately there appears to be some piston damage, and I'd guess there would be cylinder damage also but can't easily inspect that. I do not know if this is significant damage or not. Compression is equal on all cylinders at pretty much 100psi and it starts, runs and re-starts really well. I can't tell if it is down on power at all as it's probably been a problem since I had it. Can anyone tell me if the piston will need replaced or if it will require a re-bore?



I am really torn with this problem. I need the engine to be reliable as I'm planning trips to the Isle of Mann and Irish sea so will be a long way from shore if there is a problem. I don't know if a 26 year old engine is worth repairing, waiting for the next failure, but also not sure if the hull is worth re-powering.

Any comments on the fault, fix or engine quandry greatly accepted.

Phil M
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Old 12 December 2013, 11:53   #2
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If that was me I would want to strip and examine the internals of the power head, then make my decision on what to do next once i have had a look and costed it.
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Old 12 December 2013, 12:40   #3
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this is probably caused by a problem with the carb on that cylinder. you will really struggle to get it re bored as those engines have a 'blind' bore - ie you cant get a boring head to pass right through the cylinder. knowing that, i think i would see if im happy with the compression, sort carb problem (?) and just run it. as for reliability, who knows!
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Old 13 December 2013, 04:14   #4
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Thanks for the replies. Time to find a repair shop. Despite having a reasonable boating community up here, repair shops are few and far between!

Phil M
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Old 13 December 2013, 10:09   #5
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Phil M whatever you decide, weather rebuild , run as is I would strongly advise (it was part of my standard operating procedure) to strip examine, clean and re-assemble the carburetters. but if you go for a re-bore google F.J.Paynes in oxfordshire, they are just outside Eynsham, they do/did blind boring.
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Old 31 March 2014, 07:22   #6
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I've had the all clear on the bores from the local shop and they agreed that lean running may well be the issue so I have stripped the carbs off and so far dismantled and inspected the upper two, both of which seemed very clean, un-worn and in good order. I was hoping to see some residue or sludge or something to explain the problem that I could blame for the lean issue, but nothing there. One problem I did find was that the intermediate throttle link between the carbs was broken, which essentially lead to the upper throttle plate not opening when the others did, and lagging a fair bit once it did open. Could this cause lean running in the mid cylinder? Essentially, apart from at idle, the middle and lower cylinder would always be more open than the upper.

Thanks,

Phil
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Old 31 March 2014, 11:20   #7
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Piston damage

For sure when the air fuel mixture is right the mixture insulates the piston crown from the heat flame, if you get what is called incomplete combustion you get hot spots on the piston crown and the piston lands around it, this will cause the piston to overheat and this may make it contact the cylinder bore and hence metal to metal and transfer / tear some of the material off the piston.

For me I would strip the engine and for what it's worth replace the damaged piston, I would be worried that if left the piston material may trap a piston ring and prevent it from operating correctly, if this happens you are in trouble wherever you are, the flame will travel down the piston and melt it.

I would replace the piston and rings, gently hone the cylinder, re build and ensure that the air:fuel mixture was right and all associated components are correct otherwise it will happen again.

The deposits are from the engine oil that is burning and sticking to the very hot piston crown and lands, again if the deposits build too much you will create hot spots that cause the air fuel to burn before it is meant to and again the piston crown may overheat.

Hope this helps you
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Old 31 March 2014, 15:11   #8
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Just a team tip, I always set the mixtures on the carbs slightly richer than the factory said they should be.A little bit of extra oil will not hurt the obm.
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Old 02 April 2014, 08:52   #9
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FWIW the 50/60 HP clamshell uses a 0.002" bigger jet in the top carb.

Also later models had lighter weight piston with 2 instead of 3 rings to allow it to rev a bit faster to compaensate for backing off the spark advance, and also removed the thermostat / poppet valve which seems to keep it properly cool cool when running even if it does get a bit wobbly at idle after an open throttle run. (and does remiove another bit of maintenance!)

That is what I found when rebuilding my 60. Double check before you do it to yours which is a subtly differnet lump!



As for the rebuild / replace debate - I went rebuild as it was going to cost as much as to buy an unknown history used engine and I reckoned if I rebuild it at least I know it's history from "day 1". My costs included a rebore & new pistons as it was pre- ignition that killed miine.
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Old 02 April 2014, 09:30   #10
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Quote:
Originally Posted by 9D280 View Post
FWIW the 50/60 HP clamshell uses a 0.002" bigger jet in the top carb.

Also later models had lighter weight piston with 2 instead of 3 rings to allow it to rev a bit faster to compaensate for backing off the spark advance, and also removed the thermostat / poppet valve which seems to keep it properly cool cool when running even if it does get a bit wobbly at idle after an open throttle run. (and does remiove another bit of maintenance!)
There are times when I find myself in awe of a fellow nobbur's specialist (totally practical!) knowledge and find myself feeling just a tad inadequate

You, sir, fit that category.
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