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12/6/2019 7:27:02 PM
Posted: 2/10/2007 3:59:25 PM EST
[Last Edit: 2/26/2007 3:57:02 AM EST by recoiljunky]
I just bought a mid 70s Mercury 20hp outboard and a 13.5" boat.

The merc studders a little too much at low speeds. The foot also looks like it's not seated all the way either.

This is going to be my dedicated fossil hunting / SCUBA diving boat.

To make this thing reliable should I have a professional work on it, get a manual and work on it myself, or throw the thing off a bridge and get something newer?

I'm fairly handy with tools. I assembled my own AR from scratch (upper too.)

Are any specialized tools needed to work on this thing?

How much can I expect to pay to have the carb and timing adjusted? Is there anything else I should have a mechanic do?

Any good online sources of info for this motor?

Edited to add: Problem solved. The motor was running rough and would not idle. I bought a manual and a carb rebuild kit and went at it yesterday. Rebuilding the carb took about 2 hours, installing the carb took about 15 minutes and tuning the motor took another 15 minutes.

It runs like a champ now!
Link Posted: 2/10/2007 4:11:41 PM EST
Go get a manual and do it your self. I had 50's era Johnson OD and it sat for years. I got a manual and in a couple of hours it was running great. I am still using it ten years later. Be sure you dont dry start it and go get a new impeller for it. Tune the carb and you should be good to go. Those old engines will run forever.
Link Posted: 2/10/2007 4:12:58 PM EST
I would get a manual. They are pretty simple. Kinda like an old lawn mower engine. You can more than likely get all the tuneup parts for under $25 if you cross reference what else they work in. If you take it in it will be 100 because its a boat.
Link Posted: 2/10/2007 4:44:31 PM EST
Good to hear the vote of confidence. I'd rather do it myself as long as I'm not running a big risk of screwing it up.
Link Posted: 2/10/2007 6:33:03 PM EST
Old mercs are pretty easy to work on. I grew up fixing Mercs in the summer and Polaris snowmobiles in the winter.
Link Posted: 2/10/2007 6:42:21 PM EST
Don't overlook the coils, many an old outboard has been put away because it could not be made to run right and all it needed was a couple of coils. They probably cause more problems then carbs do, it's just people mess up the carbs trying to get them to run right.
Link Posted: 2/11/2007 4:24:12 AM EST
I ran it for a while in a bucket yesterday. It never ran as smooth as I think it should, but it should push me around the creek this afternoon. I'm going to try to run a tank of fuel through it before I go to making any adjustments.

It's cooling fine, the water is coming out strong and warm.

It's real hard to crank when it's cold. Most likely set to lean from what I've read.

Link Posted: 2/11/2007 4:37:37 AM EST
[Last Edit: 2/11/2007 4:39:08 AM EST by jmarkma]
Don't forget that you should replace your impeller about every two years. I know some people who will change theirs every year if they use their boat a lot. That model engine probably has a pin going through a shaft just below the engine that will prevent you from pulling the lower unit off. The models with the splined shafts are a bit easier to remove.

Putting a foaming carbon cleaner into that motor is probably a good Idea also. OMC makes a product which you spray into the carb while the engine is running then let it sit overnight. I would do that regardless with a motor that old but your carb may also be the problem.


ETA: A good stream of water is important for cooling but waiting till it is not is a sure fire way to burn your engine up.
Link Posted: 2/11/2007 1:35:30 PM EST
[Last Edit: 2/11/2007 1:52:51 PM EST by recoiljunky]

Originally Posted By jmarkma:
Don't forget that you should replace your impeller about every two years. I know some people who will change theirs every year if they use their boat a lot. That model engine probably has a pin going through a shaft just below the engine that will prevent you from pulling the lower unit off. The models with the splined shafts are a bit easier to remove.

Putting a foaming carbon cleaner into that motor is probably a good Idea also. OMC makes a product which you spray into the carb while the engine is running then let it sit overnight. I would do that regardless with a motor that old but your carb may also be the problem.


ETA: A good stream of water is important for cooling but waiting till it is not is a sure fire way to burn your engine up.


The seller changed the impeller out in front of me. This model motor does allow me to pull the lower unit off; it has a splined shaft.

I may have to give the carb cleaner a try.

I lived through the maiden voyage today. Me and a buddy ran upstream about 30 - 45 minutes. The motor runs good when you get to about 1/2 trottle, any less and it "sneezes" a lot. It's also difficult to start. It won't idle.

I've ordered a repair manual.

"Not" or "Hot"? Will a motor burn up if the water coming out of it is warm / hot?
Link Posted: 2/11/2007 2:09:13 PM EST
i have a 85 model 25 horse merc.

i have found 2 things that really helped out.

one is a product called SEAFOAM and the other is the MERCURY foam carb cleaner you can get from a dealer.

these really improved my motor.
Link Posted: 2/11/2007 3:52:49 PM EST
I'd check the plugs or possibly go to a hotter one. I have a 20 made in 69 and it can foul plugs easy if you don't mix the oil right. It sneezes alot when the plugs get crappy. Or it ould be something as simple as the idle adjustment. Best of luck.
Link Posted: 2/11/2007 4:17:40 PM EST
[Last Edit: 2/11/2007 5:28:31 PM EST by copterdoctor]
After you get your repair manual, replace the spark plugs and clean out your carburetor. It sounds like your carb idle system is crapped up. You can get good quality aftermarket parts for a lot less than at a Merc dealer. One source is Sierra ww2.sierramarine.com/minor_cat.asp?major_cat=SIERRA%20MARINE%20SECTION . You can also order Sierra marine parts through a lot of NAPA auto parts stores.
It's OK for the water coming out to be warm/hot. It's carrying away engine heat.
Link Posted: 2/11/2007 4:38:11 PM EST
I got a 64 Johnson Super Sea Horse this summer at a garage sale for my 17 foot alumacraft. I can't get that damn thing running for the life of me.

I replaced the fuel filter and cleaned the plugs and sprayed carb cleaner in the carb but didn't pull it apart. I need to find out where and what a "coil" is. I assume ignition coil like in a car but I can't tell where it is.

I also am not sure how to hook it up to the steering cable. The cable off the con seems to want to screw into the engine housing like on a newer outboard but this one doesn't have a threaded cable sheath. I need some sort of mount that goes on the transom or something....


I'm clueless about outboards.
Link Posted: 2/12/2007 2:22:56 PM EST
BTT and keep the info coming.

Someone on another board suggested I look at the points insulators. Sounds reasonable.

I'd really love to get this thing to run good. The boat is the perfect size setup for what I'm needing. Me and a buddy going diving in the Chipola River and a few other small, clear rivers in search of fossils and artifacts. We took a larger aluminum boat out in the creeks last year and it was too big to get past some of the shallow spots and it was difficult getting it over the logs.
Link Posted: 2/12/2007 2:57:05 PM EST
I have a 1940's Evenrude that stiil runs.
Link Posted: 2/12/2007 3:09:30 PM EST
Fix it yourself. you will get more satisfaction out of it that way. Plus if it breaks down
on you out on the water, you will at least know where to start hitting on it.
Link Posted: 2/12/2007 3:17:03 PM EST

Originally Posted By huggybro:
Fix it yourself. you will get more satisfaction out of it that way. Plus if it breaks down
on you out on the water, you will at least know where to start hitting on it.


That's the theory I'm working with.

Pic of the yacht:

Link Posted: 2/12/2007 3:57:02 PM EST
Looks like a fun boat for diving and fishing.
Link Posted: 2/13/2007 5:47:19 AM EST
[Last Edit: 2/13/2007 5:48:09 AM EST by jmarkma]

Originally Posted By recoiljunky:

Originally Posted By jmarkma:
Don't forget that you should replace your impeller about every two years. I know some people who will change theirs every year if they use their boat a lot. That model engine probably has a pin going through a shaft just below the engine that will prevent you from pulling the lower unit off. The models with the splined shafts are a bit easier to remove.

Putting a foaming carbon cleaner into that motor is probably a good Idea also. OMC makes a product which you spray into the carb while the engine is running then let it sit overnight. I would do that regardless with a motor that old but your carb may also be the problem.


ETA: A good stream of water is important for cooling but waiting till it is not is a sure fire way to burn your engine up.


The seller changed the impeller out in front of me. This model motor does allow me to pull the lower unit off; it has a splined shaft.

I may have to give the carb cleaner a try.

I lived through the maiden voyage today. Me and a buddy ran upstream about 30 - 45 minutes. The motor runs good when you get to about 1/2 trottle, any less and it "sneezes" a lot. It's also difficult to start. It won't idle.

I've ordered a repair manual.

"Not" or "Hot"? Will a motor burn up if the water coming out of it is warm / hot?


If your impeller fails your motor will lock up in short order. Probably a few minutes of running would do it. The stuff I am talking about is not carb cleaner. It is a foam that is sprayed into the carb while you are running the engine. It is a carbon remover.

The problem you have sounds like more of a carb problem but putting some carbon remover though the motor is not a bad idea either. Some of these old motors have points. That may be the source of your problem. I would see if you can find a repair manual so that you can see what you are dealing with. A parts schematic that they should have at the parts store should be of help also.

ETA: I see that someone already suggested checking the points. You should be able to buy a kit to replace everything.
Link Posted: 2/13/2007 6:29:00 AM EST
These things are pretty much bullet proof. If it's not completely locked up anyone can do repairs. Parts are everywhere. You don't define what it's doing when you state that it's rough at idle, small outboards idle like an old tractor just normally.

BIG +1 on the Sea Foam fuel additive. It's a yearly ceremony for me along with a new impellor and lower unit lube. I'd do this before anything else. You'll be amazed at the gunk it will clean out of a motor. Practically a rebuild in every can, not bad for $5.

Check spark plug wires and insulators for hairline cracks. The coils are potted in ceramic and may need a look-see as well. This is most likely NOT your problem though. Spray some WD 40 on them then wipe it off. The cracks will weep and give themselves away.

Set the timing and re-do the points, make sure the prop is in good shape. (for balance. The vibration a bad one will cause is unbelievable.) You should be good to go.

Don't worry about overheating. As long as a solid stream of water comes out of what my wife calls the "PeePee hole" you are good to go.

One caution. Do not EVER run that motor without a good water source to the impeller. {I see idiots doing this on boat ramps all the time) You can burn it up faster than you can say "$90 dollars an hour for labor"

It took less than 15 seconds (and the wife) to shred a brand new impeller on our boat. Pulling a lower unit off a 225hp Johnson in a parking lot is NOT fun.
Link Posted: 2/19/2007 8:37:08 AM EST
Another weekend running the motor. I keeps running better the more I run it. I probably need to rebuild the carb but I'm still holding off on that. We'll see.
Link Posted: 2/19/2007 9:28:53 AM EST

Originally Posted By Johninaustin:
It took less than 15 seconds (and the wife) to shred a brand new impeller on our boat. Pulling a lower unit off a 225hp Johnson in a parking lot is NOT fun.


Are the impellers made from hard plastic, or flexible rubber, or ???
Link Posted: 2/19/2007 10:08:06 AM EST

Originally Posted By PeteCO:

Originally Posted By Johninaustin:
It took less than 15 seconds (and the wife) to shred a brand new impeller on our boat. Pulling a lower unit off a 225hp Johnson in a parking lot is NOT fun.


Are the impellers made from hard plastic, or flexible rubber, or ???


Hard rubber. If you have just replaced an impeller and lubed it you will not do any damage to start the motor for just a moment. But if you run it completely dry you have a hard rubber to metal contact that creates a lot of heat and friction = destroyed impeller.
Link Posted: 2/23/2007 4:33:10 PM EST
Tomorrow, as soon as the hangover wears off, we'll take another test run.

The more I run it, the better it runs.

My dad used to say, "When they start running real smooth, that's when you need to start worrying. It's about to blow up."

Of course, he was talking about ragged out 4 stroke auto motors, but I fear the same may apply here.
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