Topic: 7.5hp late 50s mdl 7520 carbon coating – Antique Outboard Motor Club,Inc

This topic has 9 replies, 5 voices, and was last updated 9 years, 7 months ago by crosbyman.

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January 6, 2016 at 1:27 am #3333
crosbyman
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just wondering as to why the lower piston face would be covered with a coating of carbon while the upper piston is on bare metal "clean"

I took the cyl head off to install a save-a -thread insert on the upper plug hole and found the lower cyl face coated black

this a single line…. pump converted ……7.5 which runs great.

did the upper cyl get cleaned with storage oils but if so…. why didn’t the lower cyl face get washed out of its carbon deposits ? ❓

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January 6, 2016 at 2:00 am #29694
chris-p
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carbon buildup is on the head, or the piston?
January 6, 2016 at 2:11 am #29696
Casey Lynn
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Possible water leak into that cylinder. Not all leaks are big enough to shut off the combustion event.
January 6, 2016 at 3:15 am #29698
jeff-register
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Bingo!! Water leak from exhaust covers
January 6, 2016 at 5:18 am #29704
crosbyman
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…..the lower piston face would be covered with a coating of carbon

interesting…. I will inspect closely thanks for heads up

now that you folks mention it I think I did see some oxide

I will let you folks know if I find anything suspect but I seem to remember changing the exhaust cover plate to stainless. maybe a bad gasket …

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January 6, 2016 at 10:09 pm #29714
crosbyman
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the red oxide was just a small unpainted section of the cyl head… no harm

I’ll just do the save a thread and run the engine next summer then inspect the cyl lface for excess carbon

if it shows up again in excess on the lower cyl face I redo th exhaust cover.

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January 6, 2016 at 10:28 pm #29715
frankr
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Often neglected item: The carbon seal on the top end of drive shaft/bottom end of crankshaft. If damaged or missing, water can enter the bearing. Then the oiling system pumps it to the top bearing, where it enters the crankcase. Sometimes results in thrown upper connecting rod. People like to blame that on oil, but it was water damage.
January 6, 2016 at 10:43 pm #29717
crosbyman
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I was careful to reinstall that seal when I did the impeller …do these things break

would it help to check / inspect for water in the oiler side tube… if I replaced it with a clear transparent piece of fuel line
before i go pulling the head to check the carbon seal ?

I am now running my " pristine 9.5" mdl 9623 as a kicker. this 7.5 isn’t going to be used much anymore along with all my other hobby projects…

3.9 merc 3 xjw’s, a 4hp 4r71, a fd a qd … I just dont have enough boats to go around 🙂

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January 6, 2016 at 11:47 pm #29720
frankr
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Hm.m.m, good question. I never tried that.
January 7, 2016 at 7:00 pm #29757
crosbyman
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Well… I’ll just put one in and…see 🙂

I really like this $75 plus parts…. 7.5 hp but fishing time is limited.

Had the same problem before I retired but fishing time is still scarce 🙂

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