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1980 80hp mercury poor idle rough low rpm

zaschf

zaschf

Member
Hi all,
since last month's fixed electrical issues with the old engine all when fine, so I thought.
But it idles rough and will stall when put in gear and I can't get the engine to idle at the recommended rpm(650). It won't idle below 1000rpm.
The obvious suspects are the carbs so taken them off the engine and cleaned and checked the thoroughly. They were actually clean as a whistle and no wear marks.
Next were the spark plugs which I had replaced during my electrical fault finding. Guess what? Look at the attached photo and you'll see that they are old spark plugs. The new ones are still on the bench which I forgot to put in. Case of a senior moment.
Then I noticed something odd with the spark plugs. Number three is fairly clean and with some rust marks on it. It there water getting into number three?
IMG_0116 (Medium).jpg

And out came the compression tester and I got the following readings:
1 - 122
2 - 126
3 - 106
4 - 130
Also when I took the carbs off, water was falling out of the inlets!! I guess, that shouldn't happen.

Okay, what's the general consensus here. A complete engine overhaul or something less drastic?

Cheers,
Frits.
 
Typical with this motor is worn bores caused by running too cold (no t-stat). That causes bore wear which ruins low speed running and idle.

The water in the lower cylinders might be the water cooled exhaust piece leaking. (If so, getting the plate and its bolts off without breaking most of them is a problem.) The removed part I'm speaking of is in the lower right of the photo.

Sorry. I'd start looking around for another block.

Jeff
 

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Thank you for your reply, Jeff.
I will have a go at removing the exhaust plate and baffles in the next week or so and see how far I can take it.
But first get the power head on the work bench.
It's winter here in NZ so I'll have a few months till spring, I guess.

Cheers,
Frits.
 
Be really careful removing those bolts--they snap off easily. The best method I've found is to use an impact wrench set at a VERY low setting; let it chatter away, for minutes if necessary, until the bolt starts turning. Force it and they snap right off.

Jeff
 
I've decided to put the new spark plugs in first and take the boat out on the water again. I guess, the water problem won't go away but also not getting any worse. The new spark plugs might make all the difference with the engine hesitating.

Excellent advice using a rattle gun on low impact.
I was also contemplating applying heat to the bolts then let them cool down and that several times.


Cheers,
Frits.
 
Some people heat the bolt head red hot then rap it with a hammer to loosen the corrosive grip. That works as well.

Jeff
 
Coming back on the carburetors I took off for cleaning the following.
They were really clean but one of the issues I saw was that petrol was coming out of the front of the carburetors before I took them off.
I did check the floats height as per Mercury specs and set them accordingly. Big mistake. After I fitted the carburetors and pressurised the petrol, lots of petrol was pouring out of the carburetors overflow.
With the old carburetor floats you have to allow for some loss of float-ability because over the last 35 years or so they will have absorbed some petrol or for whatever reason they have lost it for a bit.
I ended up with the floats sitting about 1.5mm higher than the Mercury specs.
Tomorrow I'm going to set up the idle a bit better which is currently sitting at 900rpm or so.
B.t.w. the engine fired up first time after fitting the carburetors with the idle screws at 1 1/4 turn and new spark plugs.

Cheers,
Frits.
 
I've ran the motor for about two hours (half an hour in the tank and the rest at sea) with new spark plugs.
Idle still very bad but WOT goes okay.
Reading the spark plugs and it looks like only 1 and 2 are really working with 3 and 4 constantly being washed out with (sea)water. And it also looks like I've set the carburetor for 1 and 2 really rich just to keep the engine running. Perhaps that should be have a hint for trouble.
Looks like I need to get the exhaust cover off and check the baffle for holes.
 

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I checked the bottom seal and that's fine. The problem is much bigger and I do think the motor is beyond repair.
There is a hole in the exhaust baffle right in front of the number three exhaust port.
Both number 2 and 3 pistons have major sideways movement and score marks on the skirts.
The fact that it was running at all is a small miracle.
See the attached photos.
You can see that the exhaust area for number 3 and 4 looks clean and washed out while number 1 and 2 look nice and well oiled.
 

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I found a second hand baffle at a shop (Mercury dealer) here in Auckland and they have the gaskets as well.
You see, the compression is still very good so, I'll just bolt it back together and perhaps get another few years out of this motor.
At least it will give me some time to save up for another motor.

Cheers,
Frits.
 
The motor is put together without any issues.
BUT, just before I wanted to put the motor on the tail, I saw a gap of about 1mm between the exhaust extension and the tail.
That isn't normal and needed some more attention. I saw that it was mounted with just one bolt so it was easy to take the exhaust extension off.
Then I discovered that it should have been bolted down with two. The back one was broken off clean at the surface of the tail. The hole is the exhaust extension was full of salt and muck and that's why I thought it was only bolted down with one bolt...

I've now drilled a hole in the bolt large enough for an 'easy out' to fit.
Now getting it out will be the trick. Lots of wd40, heat and more wd40 and a whole lot of patience I guess.
I'll let you know how I get on.
 
And here is the power head re-assembled and ready to be bolted back on.
 

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Bolt is out with the help from the local garage mechanic and his oxy/acetylene torch.
And I found the serial number of my engine. It was right under the steering arm on the back of transom bracket.
With just about everything removed, it finally clicked.
It's 5378663.
 

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We're back on the water and the Merc is purring again.
The photo was made just before finishing the tuning.
 

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It's been a little while that I've been on the water but yesterday I went out and it wasn't nice. The motor will not idle below 1500rpm and putting it in gear stalls it instantly. Also it will wreck the gearbox, shifting at such high rpms. Once going at 2000+ rpm it's fine and there is still plenty of power, but...
I checked No 3 and 4 and the compression has gone south which explains the poor idle and difficulty to start. I always have to use the shoke when starting.
Anyway, I think I'll stop spending money on this motor. I did enjoy the whole process of fixing it up and it was a good learning process, giving me a better understanding of outboards in general and Mercuries especially.
I had really hoped that this motor would last me a few seasons and give me time to save up for a newer one, but it is not to be.
Thank you for the support I got from you guys. I'll be on the lookout for a newish Mercury and I'll let you know how I get on.
 
You carried on with this motor after seeing the scored pistons ?-------Did not take it apart to address the scored pistons ?
 
Yes, I did saw that but cmpression was okay-ish at 110psi and I didn't wanted to spent too much money (that I didn't have anyway) on it.
Buying new pistons and rings would have set me back at least nz$1000 so I thought if I can get away with it I'm okay for a couple of seasons.
 
Hate to say it, but that condition is a dead giveaway the bore is worn so badly that blow by is contaminating the intake charge, killing both idle and low speed power. The only solution is an overbore and new pistons, or another block. Sorry.

Jeff
 
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