1978 mercury 140hp problem with cylinder #6

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cuenta619

Seaman
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Aug 8, 2015
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I recently bought a boat came with a 1978 Mercury 140hp engine inliner "tower of power"
start easier and runs well before the first trip I made sure to put a new set of spark plugs
at the back of the river about 2 hours /
check the spark plug 5 plug are little dark and wet
but the spark plug Cylinder No. 6 was clean and dry with no sign of Use
any ideas guy
thank
 
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Chris1956

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Mar 25, 2004
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Sure, sounds like #6 has water infiltration, which is steam cleaning the sparkplug. Very common to those motors, and caused by leaking crankshaft seals on the lower endcap of the block.

Remember, the bottom carb feeds both cylinder 5 and 6, and if #5 is running properly, it is unlikely to be a fuel problem in #6, hence my prediction of leaky seals.
 
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cuenta619

Seaman
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Messages
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Chris1956 said:
Sure, sounds like #6 has water infiltration, which is steam cleaning the sparkplug. Very common to those motors, and caused by leaking crankshaft seals on the lower endcap of the block.

Remember, the bottom carb feeds both cylinder 5 and 6, and if #5 is running properly, it is unlikely to be a fuel problem in #6, hence my prediction of leaky seals.
Click to expand...

How I can test the water leak?
What is the correct ajust to this carb ( how many turn out ) 8n the needle
 
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Chris1956

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Not sure of any other tests for water infiltration, than seeing steam-cleaned spark plugs. You might crank it up on a flusher, with #6 plug removed. Use a rag with some oil on it over the plug hole. Water will bead up on the rag, fuel will not. Also, droplets beading on the sparkplug will indicate water as well.

Initial setting for your carbs is 1-1/2 turns out from lightly seated. It will need to be adjusted by water testing, from there.
 
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cuenta619

Seaman
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Chris1956 said:
Not sure of any other tests for water infiltration, than seeing steam-cleaned spark plugs. You might crank it up on a flusher, with #6 plug removed. Use a rag with some oil on it over the plug hole. Water will bead up on the rag, fuel will not. Also, droplets beading on the sparkplug will indicate water as well.
e 1200


Initial setting for your carbs is 1-1/2 turns out from lightly seated. It will need to be adjusted by water testing, from there.
Click to expand...

Today I. Took the bottom carb to clean but look good
Spray a little w40 inside the reed valve before that the idle was in 850 or 900 rpm in neutral now have 1200 I think I'm move the carb synchronization
How much is the idle speed in neutral out of water ?
 
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Fed

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Apr 1, 2010
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A corrosion hole in the exhaust baffle plate is another possibility.

Here's another snippet a wise old mercury guru once told me, if the lower crank seals are leaking then the motor will often idle better in the water because the water helps to seal the crankcase.
Oh, nearly forgot, NEVER lever on the endcap ears/lugs... they will break.
 
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Chris1956

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on a flusher, there is no load on the prop. So the idle speed in gear or neutral will be the same.
 
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enginepower

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Jul 5, 2014
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Another thing that can cause a clean white plug is that cylinder is running too lean and that can lead to engine failure.
 
Faztbullet

Faztbullet

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Yes but that carb has separate circuits for each cylinder. One side could be clogged.
Click to expand...
No...this is a single barrel feeding two cylinders. The white plug could be : 1) bad plug 2) bad lower seal 3)broken reed 4) not firing
 
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