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1985 9.8 hp Merc ???'s

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fish4lunch

New member
First post! Newbie.
I recently got a 1985 9.8 Merc (ser#: 6620531). Had it out once and ran great!
A buddy looked at it yesterday and said it had to be a 2 stroke. Is this motor a 2 stroke?
I ran it on high octane fuel for maybe an hour. Have I done any serious damage?
If it is 2 stoke, what is the mix ratio? 50:1? Do I use a special 2 stroke oil?
I can't find anything on the net.
 
Re: 9.8 Merc ???'s

I recently got one and it is a 2 stroke and i definately would use 50 to 1 outboard oil in it. Yes you could have hurt it, but if it had been fogged for storage the damage may be minimal. I would use an octaine additive in such an old engine. Also marine fuel stablizer. My question how do I find out the year on mine???? I have only 7 digits in the serial #
 
Re: 9.8 Merc ???'s

To do a quick inspection for damage pull the spark plug and examine the cylinder walls for scoring/blackening.
 
Re: 9.8 Merc ???'s

I believe the four stroke of that class is a 9.9, not a 9.8. Yours is a two-stroke. You need to mix two-stroke oil in your gas. As per the ratio it should be 50:1. But given that you've run it on straight gas for a long while you've likely stripped a lot of the lubrication from the moving parts, I'd run a tank at 40:1 to lube things up again.

At this point you either damaged the engine or you didn't. You can run (on mixed gas) and see if you notice any issues or you can pull the head to do a more thorough inspection of the cylinder walls. It's up to you.

When you were running the engine were you keeping it at idle for most of the hour? Did you ever give it WOT?
 
Re: 9.8 Merc ???'s

If you did damage it running straight gas you may as well scrap the engine. A brand new 4 stroke 9.9 would be cheaper than trying to fix this.

Do a compression test (cold) if you have access to a gauge - should have a minimum of 100 psi in each cylinder.

If you can't test it and/or can't get it started with mixed gas, remove the exhaust manifold and get a look at the cylinders/pistons through the port (remove 26, 27, 28, and 29 from the diagram).

For others following this and wondering, Merc did not market a 4 stroke outboard before the late 1990's (1998 I think), so definately anything older than the mid-90's WILL BE a 2 stroke and you must mix the gas....

Running an hour without oil could have fried bearings, rods, cylinders, piston rings, and pistons - so be careful if it does start you could end up putting a rod through the side of the case if you have toasted some bearings....
 
Re: 9.8 Merc ???'s

Sorry, forgot the diagram..
 

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Re: 9.8 Merc ???'s

Good news!! I put some Quiksilver in the tank (50:1), and though it took a few pulls to start (flooded it), it did start with no ugly surprises. Purred like a kitty cat.
I would trust it to take me anywhere on the water.
In fact, I'm going fishing in the morning!!!

I guess it helps to be lucky, if you're going to be stupid!

Worth mentioning . . . the little gauge on the 3 gallon tank showed I had only used maybe 1/4 to 1/3 tank. A gallon or less!
 
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