Yamaha 8hp (model 8SF, 1989) with water in cylinders

[scout-j-m]

A friend was given (or maybe "borrowed") a 1989 Yamaha 8hp, model 8SF, from a coworker of his for his teenage son to use on his jon boat. I told them to bring it over and I'd help them go over it and see if its a runner after sitting in the guys shop for years. I think the story was the guy my friend got it from had never ran it himself.

First off, its super clean. The powerhead paint looks factory with zero signs of overheating or that the bolts had ever been wrenched on. After a visual once over, I tried pulling the recoil starter and it wouldnt budge. Also couldnt turn the flywheel by hand so I knew something was bound up. Popped out the plugs to find them covered in wet rust that was so gunked up it was like mud was in the cylinder. With plugs out it turned over smothly and easily so it had been hydrolocked I guess. I sprayed in some wd40 and turned it over by hand a few times. Did a spark test and it had great spark. Did a compression test and got like 220 psi on a good gauge....I guess there was still a little water in there and the gunk buildup decreased the compressed volume and skewed the numbers???

After that we hooked it up to a good clean tank of premix, put it on the muffs and it cranked right up on the second or third pull and it actually ran really well aside from not idling down like it should. Pulled the plugs once we shut it off and they were pretty dirty from all of the gunk that had built up in the head but didn't seem to be flooded with water or anything.

Where did the water in the cylinders we found at the beginning likely come from? This motor does not appear to have a traditional head like I'm used to so I don't see a way it would have entered from the head area water jacket. I was thinking maybe the exhaust water jacket. Really though I dont have much experience with water intrusion aside from a traditional failed head gasket. Is it a risk to run it as is without figuring that out first? If we can run it, what precautions should be taken? Maybe run a richer premix to keep it lubed while it expels that junk from the internals?

 

[QBhoy]

Hi. I have two of these incredible engines in the garage. In my opinion, one of the very best outboard engines ever made. I’ve a 1993 and a 1999 model. These things are worth a fortune in good order these days..especially the later of them and in good order.
It takes more than a bit of water in their cylinders to kill one of these things. As you’ve found out.
Given the unknown history of the one you have..it can only be assumed that it’s had a swim at some point previously…or some other sort of abuse similar.
I’d throw a spoonful of 2 stroke down the plug holes and turn her over by hand a couple of times. Leave her over night or repeat the process a few times over the period of a day or something. Let her recover and soak for a while.
Then box her up with new plugs and a quick carb clean (she may have ingested a drink of water from the bottom of a fuel tank previously perhaps). Run her gently on a 50-1 mix for a while and up to temp. If she behaves well…go enjoy here for another few decades. If you’re super fussy…do another compression test on her an hope for numbers above 100 psi and close to one another.
These originally called for 100-1 mix, but I’ve always ran my two on 50-1 and change the plugs each year. I run my 3hp Yamaha Malta and any other Yamaha of that era I’ve had on that too. No doubt you’d be fine at 100-1 and some may even insist on it..but I’m for being safe on it and mix her richer. Precious things these days. Especially the short shaft models.

 

[scout-j-m]

Hi. I have two of these incredible engines in the garage. In my opinion, one of the very best outboard engines ever made. I’ve a 1993 and a 1999 model. These things are worth a fortune in good order these days..especially the later of them and in good order.
It takes more than a bit of water in their cylinders to kill one of these things. As you’ve found out.
Given the unknown history of the one you have..it can only be assumed that it’s had a swim at some point previously…or some other sort of abuse similar.
I’d throw a spoonful of 2 stroke down the plug holes and turn her over by hand a couple of times. Leave her over night or repeat the process a few times over the period of a day or something. Let her recover and soak for a while.
Then box her up with new plugs and a quick carb clean (she may have ingested a drink of water from the bottom of a fuel tank previously perhaps). Run her gently on a 50-1 mix for a while and up to temp. If she behaves well…go enjoy here for another few decades. If you’re super fussy…do another compression test on her an hope for numbers above 100 psi and close to one another.
These originally called for 100-1 mix, but I’ve always ran my two on 50-1 and change the plugs each year. I run my 3hp Yamaha Malta and any other Yamaha of that era I’ve had on that too. No doubt you’d be fine at 100-1 and some may even insist on it..but I’m for being safe on it and mix her richer. Precious things these days. Especially the short shaft models.

Thank you for the reply. I'll take your advice and throw some oil in the cylinders to make sure everything gets coated.

Since it fired right up I think I'll go ahead with a carb clean to clean up the low speed orifices and jet and then move on and do a water pump kit and then drain, pressure test, and refill the lower. I'll eventually do a compression test again just out of curiosity.