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Mercury outboards ...

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Buzz Maring
Master Marine Consultant

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USA
1762 Posts

Initially Posted - 03/23/2003 : 11:48:41

Hi everyone,

I'm hoping that some of you have experience with old Mercury outboards ... I need some help.

I have a 1977 7.5hp Mercury two-stroke with electric start. Just before I put my boat in the water last November, I spent nearly $300 to have it tuned and completely checked out. In addition to replacing spark plugs, etc., they also replaced the water pump and impeller. I am now convinced that the engine has never worked right ever since I splashed the boat.

It seems the engine runs OK for about 10 minutes ... then it begins to overheat ... at least I think that's what is happening. The engine begins to surge, then it dies. It will start again after a few minutes of cooling, start to surge again, then it dies. Obviously, if it is overheating, I don't want to keep up this routine or I'll completely ruin the engine.

For those of you with Mercury outboard experience ... is the exit for the cooling water above the water? If so, it is obvious that there is no cooling water going through the engine because there is no water "spitting" out above the water line. A few people at the marina have looked at my engine, noticed that there is no water being expelled above the water, and their conclusion was that the exit for the cooling water must be below the water line ... is it?

The other question I'm wondering about ... if the cooling water isn't circulating through the engine, about how long would it take for the engine to overheat?

I'm so mad about this I could spit nails. The engine work I paid for cost literally twice as much as I expected, took twice as long as promised, and it was shoddy work. For obvious reasons, I've got to make sure that my outboard is dependable, and I certainly can't afford to buy a new one.

Thanks for the help.

Buzz Maring
<img src="http://im1.shutterfly.com/procserv/47b3df23b3127cce9306a9b3abad0000001010" border=0>
~~Freya~~
C-25 SK/SR #68

Edited by - on

Gary B.
Admiral

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USA
969 Posts

Response Posted - 03/23/2003 : 12:27:45

Buzz: I am not the expert you seek, in that it has been a long time since I owned a Mercury. But my second boat I bought in about 1978 had a Merc 4.5 as a kicker. It most certainly had a cooling water discharge ABOVE the level of the water. Most often you will find it just under the level of the removable engine cover. In fact, if you take the cover off, you should be able to see the tube under the power head. It makes no logical sense to me to have cooling water go directly into the water, although exhaust gasses often do....

If you sail in salt water, may I make a suggestion? Buy some cheap pipe cleaners and run one up the tube every so often. Sometimes engines clog up with salt deposits, and need to be cleaned out, much like some human arteries do with cholesterol!

Have you tried doing an internet search, or getting ahold of an older manual?. I sure could be wrong, but, if it was my engine, the pipe cleaner would be my first try.....Good luck.

Gary B.
Encore! #685 SK/SR (Yamaha 9.9 powered)

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 03/23/2003 : 12:34:34

The telltale cooling stream should give you an indication that the water pump is pumping water. Is the lower unit far enough in the water so that all of the water intake ports are underwater? If it isn't and air is getting into the system, then this could prevent water from being circulated into the engine.

Additionally, damage could be done to the water pump impeller if the engine was run without cooling water(say for testing) even for a few minutes.

I would replace the thermostat if it is the original thermostat. BoatUS lists a thermostat for a 7.5hp Mercury with serial numbers 5226935 and up.

<img src="http://www.catalina25-250.org/c25sm.gif" border=0>Don Lucier<img src="http://www.catalina25-250.org/c25sm.gif" border=0>
<img src="http://im1.shutterfly.com/procserv/47b2cc32b3127cce92e981259d130000001010" border=0>
North Star SR/FK

Edited by - on

ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 03/23/2003 : 13:44:28

Every outboard I've operated had a "pisser"... a small indicator stream of water that shows the powerhead has coolant pressure.

Yes, those tubes do get clogged... the pipecleaner treatment is not a bad idea. I've also found that holding your finger over the end of the pisser tube for few moments usually will back pressurize the system and let some water through.

Another thing to check.. is your gas tank vent closed or plugged? (this depends on engine/tank design)

If the vent isn't open, the motor will run until it can't pull fuel against the tank vacuum, then it will quit. After a few minutes, the vacuum leaks down and the motor will run again.

Is your fuel premixed ? Got oil? Proper ratio? Not stale ?

If you've run a 2 stroke engine to the point of stoppage (seizure) due to overheating, you've probably sustained cylinder and ring damage.

With a catastrophic overheat, you'll usually be able to tell something bad has happened as a seized engine usually has to be 'broken free' before it will turn over.

Some other indicators:

If you pull the sparkplug on a overheated 2 stroke, it will often show metallic deposits.

If you open the engine cover after it stops running... that puppy will be VERY HOT if it's not getting coolant. You'll know if it's so.

Good luck.

Currently maintaining two holes in the water...'77 Venture 23 and new to the family,
'78 Catalina 25

Edited by - on

Ben - FL
Admiral

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880 Posts

Response Posted - 03/23/2003 : 20:01:38

Sorry to hear about your engine difficulties. I know what you mean about be mad. We have a small fishing boat also with a 50hp Johnson. Took it to the shop that sells the boats to get it to run right. Took it back two more times. Each time was more money. Totaled abut $900. Finally took it to a place at the beach in a self storage unit run by a father son & cousin trio. They said the carb had never been rebuilt, and neither had the oil pump. I believe the big guys just ripped me a new one. I'd still like to tie a concrete block to that guy and throw him in the river. Wish I had the time to take a course on marine mechanics. I used to work on motorcycles all the time. Can't be that much different.

Safe voyages,
Ben, FL s/v Chick-a-pea C250wk
<img src="http://www.members.aol.com:/benraye/pictures/fullsail.jpg" border=0>

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djones
1st Mate

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USA
56 Posts

Response Posted - 03/23/2003 : 22:00:13

There is another possiblility. If the engine becomes fuel starved, it will run lean in the last few seconds, and appear to surge, then quit. You can simulate this by pulling the fuel line and see if the same thing happens after you restart the engine. If your engine has the tell-tale water indicator stream coming out, and doesn't actually appear to be overheating, then check the fuel line, filter, etc. for air leaks or blockage. Does your fuel line need to be reprimed with the bulb before it will start again?

-Don Jones '83 C25 "Swept Away" SR/FK

Don & Freda Jones
"Swept Away" '83 C25 SR/FK
Charlotte, Southport, NC

Edited by - on

djones
1st Mate

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USA
56 Posts

Response Posted - 03/23/2003 : 22:03:02

Please disregard my earlier post. You said there was no water stream coming out. That is definitely a cooling problem.

-don

Don & Freda Jones
"Swept Away" '83 C25 SR/FK
Charlotte, Southport, NC

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JimB517
Past Commodore

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USA
3285 Posts

Response Posted - 03/24/2003 : 13:55:53

Check the water pump by all means. It is easy to do. There should be some bolts right above the water intake. All you have to do is drain the oil from the lower end and then drop those bolts, pull down the lower housing (with the prop). The main drive shaft goes right through the middle of the water pump and should be right at the level of the intake.

However, "surge and die" syndrome is a symptom of running out of fuel. Check the fuel filter and tank. Make sure there are no obstructions. If the motor is overheating, you should be running along and all of a sudden it should make a loud roar and come to a halt in a cloud of steam. After a few minutes it should restart, then repeat. Another sign of overheating is peeling paint around the head gasket or other signs of trauma there.

For curing overheating problems the usual fix is : water pump rebuild kit, replace thermostat, replace head gasket. Should be less than $100 if you do it yourself. I'd bet that your pistons, cylinders and rings are ok. When you have the head off, check all passages and clean them as much as possible.

Just be careful and don't break off the head bolts when you try to extract them (like I did). When you re-bolt things together apply waterproof grease to every bolt.

Indiscipline 1978 FK #398

Edited by - on

Steve Milby
Past Commodore

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USA
5851 Posts

Response Posted - 03/24/2003 : 13:59:27

I have a 1978 Merc 7.5 and had a similar problem. After I had the impeller replaced, the motor overheated quickly each time it was started. The mechanic finally figured out that a piece broke off the old impeller and got into the water system and clogged it. In order to get the piece out, the mechanic said he had to remove the power head.

Steve Milby "Captiva Wind" C-25 T/FK #2554

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Buzz Maring
Master Marine Consultant

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USA
1762 Posts

Response Posted - 03/24/2003 : 15:31:30

<BLOCKQUOTE id=quote><font size=1 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>
I have a 1978 Merc 7.5 and had a similar problem. After I had the impeller replaced, the motor overheated quickly each time it was started. The mechanic finally figured out that a piece broke off the old impeller and got into the water system and clogged it. In order to get the piece out, the mechanic said he had to remove the power head.

Steve Milby "Captiva Wind" C-25 T/FK #2554
<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>

Steve ... do you know if your outboard has an above-the-water exit for the cooling water? Yours is so close in age to mine, they might be almost identical. It would be a BIG help if I knew whether or not I should be seeing the cooling water spitting out above the water line ... there's no water spitting out anywhere that I can see.

Buzz Maring
<img src="http://im1.shutterfly.com/procserv/47b3df23b3127cce9306a9b3abad0000001010" border=0>
~~Freya~~
C-25 SK/SR #68

Edited by - on

n/a
deleted

Members Avatar

163 Posts

Response Posted - 03/24/2003 : 16:24:44

I used to have a 1975 Merc 7.5 hp. It definately had a relief port above the water line, on the underside of the head cover. I had a problem with little pieces of neoprene from broken water pump vanes plugging up the system. A thermostat that is corroded shut could also be a problem. Good luck.

Edited by - on

Steve Milby
Past Commodore

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USA
5851 Posts

Response Posted - 03/24/2003 : 16:38:14

Buzz,

The cooling water comes out in two places. The Merc 7.5 has a "pisser" that comes out a small hole located on the aft, starboard side of the motor, just under the bottom edge of the engine cowling. The purpose of that is to let you know that the water pump is working. The fact that no water is coming out through that hole doesn't necessarily mean that the motor isn't pumping water. Usually the hole is just plugged up by a small insect or spider that gets in it. If you remove the engine cowling, you will see a small black hose that is attached to the metal part of the aft, starboard side of the motor. If it is clogged, you can clean it out by pushing a pipe cleaner or a piece of wire through the hole and also into the hose.

Most of the cooling water goes out through a hole in the center of the propeller hub. I put my motor on a motor stand, and put the shaft down into a big plastic garbage can. Then I fill the can with water just above the cavitation plate and start the engine. (Don't put it in gear and rev it up, unless it's Saturday night and you are ready for your bath.) With the motor in neutral gear, you should be able to see if water is pumping out through the propeller hub. It should pump turbulent water through the propeller hub.

Steve Milby "Captiva Wind" C-25 T/FK #2554

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