1978 Mercury Leak Near Propeller

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
cleep1700
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Location: Lake Saint Louis, MO
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1978 Mercury Leak Near Propeller

Postby cleep1700 » Sat Sep 23, 2017 4:45 pm

A couple of questions from a newbie:
After being on the water recently on my '78 15 Sport, the next day there was a black liquid that had leaked on the floor of my garage from my 1978 50 hp Merc. It seems to be originating from behind the prop. I'm not real mechanical. What am I looking at: New gasket? Seal? Easy to do or is this a "take to a mechanic" job? and if so what kind of cost am I looking at?

Also I remember there was a discussion about the pros and cons of stainless steel props. I'm using one, should I?

Appreciate your help as always.
Craig in hot and humid Missouri

Jefecinco
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Location: Gulf Shores, AL

Re: Leak near prop

Postby Jefecinco » Sat Sep 23, 2017 7:04 pm

There is a good chance the oil is leaking due to a worn propeller shaft seal. I would recommend a shop if you don't have a repair manual. If you are handy a good manual is worth buying for the future.

A stainless steel propeller is almost certainly a better choice.
Butch

frontier
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Re: Leak near prop

Postby frontier » Sun Sep 24, 2017 8:53 am

Pretty normal in an older 2-stroke to have black drips near prop. Usually a mixture of exhaust and unburned gas/oil mix. Just put a paper towel on your garage floor under prop.

Jefecinco
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Re: Leak near prop

Postby Jefecinco » Sun Sep 24, 2017 9:51 am

Frontier,

Without being able to see or touch the "black liquid" or do a finger test of the prop shaft with the propeller removed it's pretty difficult to diagnose Craig's situation. Is the engine new to him? Don't know, but if it's an engine he has had for awhile he probably wouldn't have asked if the black liquid has been seen frequently. Getting some monofilament fishing line wrapped around the prop shaft is common as is monofilament damage to the seal.

A slow lower unit oil leak is not a terrible thing. The only problem is that if oil can leak out water may be able to leak in. If it's a leak I would start by replacing the prop shaft seal and changing the lower unit oil.
Butch

jimh
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Re: 1978 Mercury Leak Near Propeller

Postby jimh » Sun Sep 24, 2017 4:03 pm

The black stuff may be just some exhaust residue mixed with cooling water.

If there is a propeller shaft rear seal leak, repair is not a do-it-yourself job for someone who is "not mechanical."

cleep1700
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Re: 1978 Mercury Leak Near Propeller

Postby cleep1700 » Sun Sep 24, 2017 4:10 pm

Thanks, fellas, appreciate your knowledge.
And thanks Jim for moderating this site.
Craig in terribly humid Missouri

Yellowjacket
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Joined: Mon Jul 31, 2017 9:33 am

Re: 1978 Mercury Leak Near Propeller

Postby Yellowjacket » Mon Oct 02, 2017 7:24 pm

It could be the lower unit rear seal, but most likely it is "spooge", a mixture of oil and unburned fuel.

The lower unit lube can easily be checked, pull the plug and drain the oil and check for water in the oil. If it isn't contaminated, refill with fresh oil. You should drain and check the oil in your lower unit at the end of every season. If you have water in the lower unit you run the chance of it freezing and cracking the lower unit, or it will result in rust and ruin the bearings if it sits in there for months. If the oil is good you just have some oil and fuel mix that is collecting in the exhaust system.

Depends on how you were running the motor, how much oil you were mixing in, more oil is better than less, but too much can foul plugs and put a lot of carbon in the exhaust. If you were running a lot of low speed or idling these engines run rich and you can get some excess fuel and that can drool out. Also I've taken a number of them apart and they tend to build up carbon in the exhaust cover and in the exhaust pipe that runs down into the lower unit. If you're running rich you can dump a good bit of fuel into those two places and it will result in some dripping of goop out of the bottom of the prop if the engine isn't run with some power before shutting down.

I Have 3 of those engines and from time to time the floats can get out spec, or the rubber tips on the floats go bad from ethanol in the fuel and the engine will run slobbery rich at low speeds. To check for that, take off the front cover and wrap and hook up the fuel line and squeeze the bulb until it gets hard. Then squeeze it some more and see if you any fuel dribbling out from either of the carbs. If the floats are set too low or the rubber tips are gone, the fuel pressure will push fuel past the float needle and fuel will overflow from the float bowl and that could well be your problem. At higher speeds this doesn't seem be a problem, but after a prolonged idle or trolling can result in plug fouling and spooge coming out of the exhaust.

Real early versions of these needle and seat sets were brass, later versions of these needles have a small rubber tip that seals well, but they were designed and built before there was any such thing as ethanol in the fuel and these can deteriorate over time. If the floats fail the pressure test you will need to take the carbs off of the engine and open the bottom of the carbs and check the float levels. Carb rebuild kits are not expensive and these have replacement needles. Setting the float levels isn't a big deal, but if you aren't mechanically inclined you'll need to take it to a mechanic and have the carbs checked. Also the fuel pump diaphragm can leak fuel into the cylinder if the diaphragm is bad.

At any rate, if you have one of these you really shouldn't be using ethanol laced fuel in these motors as it can eat the fuel pump diaphragms, the hoses and the rubber tip on the float needles.

cleep1700
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Re: 1978 Mercury Leak Near Propeller

Postby cleep1700 » Wed Oct 04, 2017 7:03 am

Yellowjacket,
I understood much of what you wrote and appreciate your wisdom and knowledge. Some of it went right over my head, the result of way too much time in college and way too little time tinkering. I rarely use fuel with ethanol and will drive miles to get real gas. Thank you for taking the time to help me.
Craig in Lake Saint Louis where it is slowly turning into autumn...