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Cracked cylinder walls? 1982 25hp

T

the_beave

Member
Will an outboard run with both cylinders cracked? I have a 1982 Mariner 25hp that was given to me. At the time it had a cracked water jacket that I repaired and then had to replace the water pump and impeller! Finally got it running this weekend and on the muffs it runs great but when it is immersed in water it runs but really lacks power! I pulled the plugs and both cylinders have water in them! I am wondering if when the water jacket cracked ( which I assume was water inside that froze!) could both cylinder walls have cracked and if so would it still run? I know that it could also be the water jacket gasket or the exhaust gasket (which looks like there might have been some leakage around it as it looks like there might be some rusty crud around it but no sign of water that I can see) I haven't been able to do a compression test yet as I don't have a tester but am gonna try to get one!
 
Re: Cracked cylinder walls?

Scrap. Someone overheated the heck out of that poor thing. Better start looking for another power head on eBay.

Jeff
 
Re: Cracked cylinder walls?

That particular Merc has no head gasket to let water in. If it runs okay at no load, but not at full throttle, the block is cracked.

Jeff
 
Re: Cracked cylinder walls?

Head Gasket part #81555M
is that not a head gasket? gonna check my compression this weekend what should I be looking for?
 
Re: Cracked cylinder walls?

That might be a true Yamaha product (not one they designed for Merc). If so, it has a removable head and a possible gasket leak.

You might spot the problem with a compression check. Look for at least 100 psi on BOTH cylinders.

Jeff
 
Re: Cracked cylinder walls?

Ok so I did the compression tes today and this is what I got: on the top cylinder I tested it 3 times and I got 100 each time, on the bottom cylinder I ended up testing 5 times and I got 180, 170, 150, 140 and 140! I've never done a compression test before so I don't know what this tells me but I don't think it's good lol!
 
Re: Cracked cylinder walls?

I think I will pull the head tomorrow and take a look! I don't have a replacement at the moment but it will let me take a look inside!
 
Re: Cracked cylinder walls?

Good. Check the gasket super carefully for any blown through spots.

Jeff

PS: If this motor has removable heads the bolts will be LARGE--probably take a 13 to 15 mm socket. But if the head does not come off, the water jacket cover bolts will be small--about a 10 mm socket size. If the later, do NOT remove them! It would do no good, and you might twist one off.
 
Re: Cracked cylinder walls?

I took it off today and it looks really bad but no obvious real blown thru spots but there does look like spots that were leaking into the cylinders! There is a spot near where the water jacket is cracked that looks kinda bad but the cylinder walls themselves are fine! I am probably gonna put it away for the winter and get a gasket for next spring!


:( UPDATE: Upon further inspection I did find what I did not want to see, a crack inside the top cylinder! It was found with the piston in and looking in thru one of the holes in there! It's not on the wall where the actual piston slides but it does allow the water to get into the piston! I guess I'm to the point that I'm gonna tra and part it out and look for something else over the winter!

Thanks for helping!
 
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