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Old 10 July 2009, 16:18   #1
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Mercury 200 20 hp

Aquired cheap mercury 200 20hp today but it has a small problem.
The impeller has been changed no problem engine runs fine( on muffs) BUT when you imerse it and cool from the pump then the bore floods and cuts her out
Am I right in thinking that it is more than likely to be the gasket at the top of the shaft below the head
Head is absolutly fine no leaks (so I`m told)

if it is this gasket will locktite instant gasket be ok with left in place the paper one or not if not will i have to buy the gasket (can i get them seperatly) or can i make one from gasket paper

If my suspicions are wrong any one got any ideas wher to look for the fault
i have changed the metal plate on the side as the old one was er in a bad way
Or have i bought a bit of scrap

Cheers Allen
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Old 10 July 2009, 16:37   #2
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it might be the exhaust plate gasket leaking, but at that age it is just a pile of junk not worth spending money on if you get it running properly it will still be an old obm worth pennys if that
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Old 10 July 2009, 17:51   #3
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when you say the exaust plate gasket do you mean the plate on the side tiller control side oblong plate with top corner angled??? or is it somewhere else
plus when you say that age any ideas how old
(its the thunderbolt ignition one if thats any help LOL i aint got aa clue where to look for the serial number or plate again ideas would be mosst helpful)

Cheers Allen
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Old 10 July 2009, 19:04   #4
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What exactly do you mean by 'the bore floods'?


Describe exactly what happens.
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Old 11 July 2009, 01:32   #5
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Quote:
Originally Posted by Nos4r2 View Post
What exactly do you mean by 'the bore floods'?


Describe exactly what happens.
Run it submerged in a runinng position and the cylinders fill with cooling water and cut engine out but cant for the life see where it comes from
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Old 11 July 2009, 02:14   #6
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Originally Posted by AllenF View Post
Run it submerged in a runinng position and the cylinders fill with cooling water and cut engine out but cant for the life see where it comes from
It won't be the head gasket as it doesn't have one-it's a blind bore. What looks like a cylinder head is actually a water jacket cover.

Bear with me here, but how do you know it's filling with water? Which cylinder is getting it in?
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Old 11 July 2009, 02:33   #7
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mmmm bear with me here cos i know NOTHING about these things
it appears to be both .. when i take the side cover plate that the (I think is the throttle ccable) attatched to both pistons look wet.
It has had a new impeller fitted (Not by me)and if my suspicions are right you basically have to take the shaft out and its in there somewhere. when i look carefully underder the main top section there is a gasket between the two bits . could it be that that has blown
Or as said earlier a leaking exaust plate ??? is that the plate I`m taking off its held on with 7 or eight bolts and the remmote thottle cable is also on it ( the side OPPOSITE the gear selector
(mind you people keep on about a pisshole ... where is it i assume the water goes in near the prop up the shaft round the head and out
I think the exaust in central at the back (i can see through when i take that dammed plate off lol) but wher is the water outlet mabe i can backflush it and find the leak that way ???? or not
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Old 11 July 2009, 02:39   #8
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Hmmm...

Wet in what way? Is there droplets of water on them or any creamy emulsified oil on the spark plugs?

Your impeller is on the shaft at the junction of the lower unit,accessable by dropping the gearbox off. An impeller change can't cause cylinders full with water-no matter how badly it's bodged.
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Old 11 July 2009, 03:15   #9
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k i just been given the old INNER cover plate that covers that old hole and its in a pretty bad way. corroded away. the new one is good the outer plate is in good condition but the gasket between the plate and the jacket ( thick thing ) has a hole in it alond the top edge of the jacket channel (if that makes sense)
found the pisshole a little 3/8 " pipe that comes out ot the top of the jacket so when i get home from work ill push some coloured water backwards and see

As far as I can tell it was COMPLETLY flooded with water which indcates a gasket somewhere doesnt it
either way i dont think it terminal do you ??
Can i make a gasket for jacket to plate or do i HAVE to buy one got some thick gasket paper and joint compound
the last numbers on serial are 92 do i take it its a 1992 model.. if so are parts readily availible still.

when you say the lower unit do you mean the section above the prop or the WHOLE shaft assembly ( thats the gasket i was suspecting but now i found the cover plate gasket in that condition I`m not so sure

Next thing is to change it back from remotes to tiller control how easy is it and are the tiller handles sstandard or not

Cheers Allen
PS sorry to be a pain and appear thick here but im an engineer not a mechanic LOL
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Old 11 July 2009, 03:22   #10
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If it's completely flooded, just bin it. It's terminal-unless you've done something silly and raised the prop higher than the powerhead immediately after you ran it and water has run back up the exhaust.

Gaskets need to be genuine-some are heat resistant.


Running coloured water up the pee hole won't help. It'll just run out of the exhaust in the middle of the prop.
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