Old (1967) merc 3.9hp overheat?

I just bought my first outboard from guy on craigslist. It's a 3.9hp 1967 merc. It started right up at his house and then I took it out on my boat with no issues. Then I took apart the carb to clean it, also replaced both fuel lines because they were cracked and leaking when I took the cowl off.

Now this is the first outboard I've owned. I've used several others before and never had a problem. THis is what happened:

I was out today fishing and I rode the motor at low-medium speed for about 10 minutes through some shallow areas. Then I shut it off for about 3 hours while I anchored and fished. Then I started it back up and started moving along slowly just checking out the lake I was on. All of a sudden the motor dies and I notice a lot of smoke coming out of the top. I assumed it overheated so I let it sit for about 30 minutes. I started it back up and it ran just great for another 20 mins or so when I made it back to the launch.

I'm just wondering if this is indeed an overheat and if it could have just been caused by running in shallow water and kicking up a lot of silt. I did run it again in shallow water on the way back and had no issues... Also, I replaced the fuel lines with tygon because I could not find original lines anywhere and I noticed they are a little larger diameter than the originals. Could this cause an overheat?

Is it a real pain to pull the lower unit apart and check the impeller if I've never done it before? I did take the carb out and completely disassemble/reassemble it without ever doing it before.

Are there any other maintenance things I can do to prevent this or other issues with this old motor? I know these things can last forever if you take good care of them.

Thanks in advance!

Re: Old (1967) merc 3.9hp overheat?

ejbatenb said:

Is it a real pain to pull the lower unit apart and check the impeller if I've never done it before? I did take the carb out and completely disassemble/reassemble it without ever doing it before.

Are there any other maintenance things I can do to prevent this or other issues with this old motor? I know these things can last forever if you take good care of them.

Thanks in advance!

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ejbatenb,

You need to replace the impeller. Simple 20 minute job to replace with a 9/16" wrench, a 1/2" socket and a flat blade screwdriver and nothing will 'explode' when you drop the lower unit. Smear some grease on the shaft splines when you put it back together. I like to drop the LU with the motor vertical, like on the boat when it's out of water or on a sawhorse. Makes it easier to stab/mate them back together. A new impeller will run you around 10 bucks. Mercury used 2 different shaft diameters on these motors through the years so have serial number in hand before you order.

Re: Old (1967) merc 3.9hp overheat?

River Boy, thanks for the info.

I took it out last night and had the same issue. This time I made note to check the water pump indicator (pee hole) to see if it was exhausting water. It was "spitting" water out intermittently with a lot of exhaust smoke, not a steady stream of water. This makes me believe it is indeed the impeller. I'll update this post if I have any trouble doing this fix.

Also, what kind of grease should I use?

Re: Old (1967) merc 3.9hp overheat?

Okay so I just took the lower unit off, very easy indeed. I found the impeller inside the water pump housing. It only had about 2.5 flanges left on it. The rest were strewn about inside of the pump. So I definitely found my overheat problem.

However, another question arose. I thought the lower unit was supposed to be filled with oil. I opened it up and there was just this yellow, slimy, bubbly residue covering everything. Only about a few tablespoons dripped out when I took the unit apart. Should I clean this stuff out and then refill it will a certain type of gear oil? If so, what type for this motor?

Re: Old (1967) merc 3.9hp overheat?

ejbatenb said:

Okay so I just took the lower unit off, very easy indeed. I found the impeller inside the water pump housing. It only had about 2.5 flanges left on it. The rest were strewn about inside of the pump. So I definitely found my overheat problem.

However, another question arose. I thought the lower unit was supposed to be filled with oil. I opened it up and there was just this yellow, slimy, bubbly residue covering everything. Only about a few tablespoons dripped out when I took the unit apart. Should I clean this stuff out and then refill it will a certain type of gear oil? If so, what type for this motor?

Click to expand...

Make sure you dig all the pieces of the old impeller out of the passages before you put it back together.

Any kind of heavy grease, something like what you'd grease wheel bearings with ya know. You're just making sure the shaft doesn't sieze to the crankshaft when you take it apart next year to see if the impeller is in good shape.
You will not wear out those lower unit bolts and screws taking them off to look at the impeller.

Yes yes, definately change the grease in the lower unit. Let all that old crap drain out and refill it with 80 90 weight Marine lower unit grease. The stuff in the the blue bottle from Wal Mart is good enough or any brand will do. With motor upright, squirt grease in the bottom hole untill you see grease comes out the top 'vent' hole. Then put both plugs back in.

Sounds like that little motor has been neglected. 3.9 Merc would be a good one to learn on, they're about simple as a motor can get. Why don't you get you a manual for it. You can buy a manual for it here on I Boats or used on Amazon.

River Boy

Re: Old (1967) merc 3.9hp overheat?

So I snapped some pictures of this yellow/cream colored foamy residue I was talking about. It looks to me like an oil/water mixture and it smells to high heaven like 2 stroke gas. It's all over inside the motor shaft case and pooled up around the water pump housing and water intake tube (see photos).

Any idea what this is, what caused it, and how to fix it?

I drained the lower unit oil and it was black but didn't have any metal shavings or water in it. Also, it looks like there is a lot of dirt (sand/silt) all over the shafts and parts inside the case (see third photo).

Thanks again for the help.

Here's the area under the water pump intake tube.


You can see it's pooled up pretty well underneath down near the prop.


Pooling near the water pump/shifter shaft.

Re: Old (1967) merc 3.9hp overheat?

Lower crankshaft seal probably. IT and LU seals. The mix drains down through those 44 year old seals right into the LU. Gets mixed up with water to create that frothy looking stuff. Do a search on replacing seals, plenty will come up. Good Pics.

Re: Old (1967) merc 3.9hp overheat?

Well I replaced the impeller, flushed all water tubes, and popped it back together. As soon as I started it no water started coming out the indicator hole. So I revved it up a bit and water started spitting out of the hole with exhaust smoke. And I mean a very very little amount. Then all of a sudden water started spraying in a stream out of the location indicated in this picture (sorry the quality is bad, it's my phone).



So water has to be getting pushed up into the motor because it's coming out of this spot right near one of the bolts in the bottom cowl. I'm pretty sure the bolts go in to the block. As I mentioned, I hooked a water hose up to all the tubes and water was streaming out of the "pee" hole. There were no clogs. I have the service manual and of course it says nothing.

Is it a crack in the cylinder where water is supposed to flow around? Is this motor even worth trying to fix now?