1979 Inline 6

[nathanhooper]

I wrote a while back about this motor, 1979 90hp Merc. Turns out I am buying it from the guy. When I first tested it, for lack of a better word, I could not get the thing to turn very easily by hand. It just seemed real ruff. I was a little leery about it, so I did not pursue it too hard. He is made me a good deal on it, so I decided to go for it.

I have the thing at my house now and decided to take the shift and throttle cables off, making sure that it was in neutral. Everything works as it should there. Then, while I knew it was in neutral, I tried turning it again by hand. Same thing as before, very hard to turn with no real "thump" of the motor turning over.

I decided to take the plugs out and try it. WOW. Turns like butter. You can hear the "hiss" and "shwoop" of the air running in and out of the cylinders, along with a noticeable "thump" now. I guess all along it was just the compression that was causing the stiffness.

I did not check the compression yet, started raining on me before I got to that point. The piston heads looked amazingly clean, there was some soot on them, but they were definitely not seized or beat up by anything. You could see the very noticeable aluminum color shining through. Which also would lead me to believe that the motor has not over heated before. Or is it just an assumption that because the pistons have a "normal" aluminum color that it has not been over heated? I know that on the motor I am working on the pistons were a lot darker in color on the heads and even the skirt of the one that looked as though it had seized at one time.

Does this sound normal, the whole compression issue, with plugs in and out? The starter can barely turn it a quarter of the way before it bogs the starter down. There is some major compression with those plugs in. Is that the way these motors are built? Is there a way to make sure the starter is just not weak? I am thinking it is a very low hour motor. It just seems very clean, except for the dirt dabbers that found a home over the last several years of it sitting outside.

 

[ClassyGlassy]

Re: 1979 Inline 6

make sure you have enough CCA, Cold Cranking Amps, to turn the motor over.

Do your compression test WITHOUT the spark plugs in the motor. A variation of 15psi from cylinder to cylinder is out of spec........some will tell you 10psi, but the manual states 15!

 

[Jacket4life]

Re: 1979 Inline 6

My manual says 10%. but it's pretty much the same.

Sounds to me like the battery needs checked first, then possibly the starter. But batteries are always suspect in my dealings with boats! Esp. if you don't know the age.

 

[nathanhooper]

Re: 1979 Inline 6

Well, here is another question. Is it ok to jump the starter directly, using my truck battery while the truck is running? Is it ok to just hook the jumper cables up one to the grounding bolt on the starter, and then tap the positive bolt on the starter to bump it? Or is it best to run through the wiring, using the key, or does it even matter?

These questions are all in conjunction with pressure checking of course. Got the manual for it today, I am going to look it over real good before going to deep. One thing I did notice is that the front "plate", where the manual choke is connected to, has a spot on it that looks like something was placed there when they painted, then removed. The holes all around it have not had anything attached to them, not a scuff mark on them, but it just seems weird.

I will take pictures tonight hopefully of the motor and see if anyone can let me know if there are things missing that I am unaware of.

 

[Chris1956]

Re: 1979 Inline 6

You can jump it directly to the starter if you are inclined. The truck does not need to be running.

It sounds like if your battery is good, your wiring is corroded. A lot of motors of that age have crumbling wiring harnesses. These can short out ign components. If the compression is good, you might see if you need a new engine harness.