1978 70hp Mercury Outboard Water Around Spark Plug Issue

austin_colby

Cadet
Joined
Apr 23, 2007
Messages
8
I just replaced the impeller on my '78 Merc 700 and the water jacket cover seems to be leaking around all three spark plugs. It never did this before the new impeller but I noticed quite a bit more water pressure from the pee hole now. After reading about 20 or so posts about this common problem, I have come to the conclusion that I would like to either a) put some marine quality high temp silicone around the cover or b) just leave it alone. It doesn't appear to affect anything and the motor runs just fine, just a drip here and there from the cowling cover. Any advice would be appreciated.

-Austin
 

Yepblaze

Lieutenant Commander
Joined
Jun 1, 2001
Messages
1,686
Re: 1978 70hp Mercury Outboard Water Around Spark Plug Issue

You are correct in assuming it can be let leak without much problem. (other than erosion making it worse after time.

I have in the past, cleaned around base of spark plug real good with wire brush and solvent with plug in place.

Blow dry.

Then remove plugs, (usually use new ones at this point).

Take the plug and with washer in place squeeze out a 1/8" bead around the threads just below the washer.

Now the hard part, WAIT a few minutes for the silicone goop to start to skin over. About 3-5 minutes on a warm day in the shade.

Then screw in plugs, the silicone should goosh out into the clean cavity and seal it up. Wait a few hours before using it to let the goo dry.
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Re: 1978 70hp Mercury Outboard Water Around Spark Plug Issue

You answered your own question. Yes your pressure is now higher resulting in the discovery of a leaking water jacket cover, probably due to corrosion at the gasket/cover interface....common problem.

No it hurts nothing at this point if you don't mind a little water residue. Over time, if it continues to leak it will get worse, because corrosion will get worse and one day you'll have to change the gasket......really quite simple.....as long as your screws cooperate and come out like they are supposed to.

Since others seem to have a successful repair with silicon go for it. What the heck.

Mark
 

Plainsman

Rear Admiral
Joined
Apr 2, 2006
Messages
4,062
Re: 1978 70hp Mercury Outboard Water Around Spark Plug Issue

I learned the hard way and insisted on changing the seal. I broke something like 10 bolts and it cost over $200 to get them out.

Use the marine grade sealant to stop the leak.
 

dbecker1

Cadet
Joined
Aug 2, 2011
Messages
10
Re: 1978 70hp Mercury Outboard Water Around Spark Plug Issue

i took my cover off and placed a piece of emery cloth on a flat surface did th figure 8 motion till the surface appeared to be flat installed a new gasket and used silicone around plug holes and and along the top and bottom edge of the fat part of the gasket were the plug holes are had no problems since
 
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