1978 Johnson 70hp. Bad head gasket or exhaust gasket?

[BonairII]

Found a 1978 Johnson 70hp w/ controls on CL today. Came with a new prop and brand spanking new fuel pump. 1/2 of the skeg is missing, but that's not a huge deal. Seller says he started motor in the spring and was getting water in bottom cylinder, so he pulled the motor off his boat. He just wanted motor out of his garage. I paid $100 for all of it, so it's a great deal IMO.

I pulled the plugs and the bottom electrode does have some white crud on it(caused by water?). All spark plugs look brand new, so the white crud on the bottom plug is the only clue I have. Seller said it was a bad head gasket. He said he had same problem with another 70hp motor years ago and a new head gasket solved the problem.

Did a compression check and got 100-105 for all cylinders, so I'm doubting it's a bad head gasket with the compression numbers being so close to each other. I'm guessing it's a leak in the exhaust.

Here's a link to the exhaust housing etc. What parts/gaskets are prone to leaks on this motor?

http://www.marineengine.com/parts/j...turer=Evinrude&section=Cylinder+And+Crankcase

 

[Joe Reeves]

Re: 1978 Johnson 70hp. Bad head gasket or exhaust gasket?

You mention a 80hp in your post. However, the way you speak of the problem (bottom cylinder) my guess is that's a typo and you really have a 70hp as per your post title.

Water enters via the cylinder head, exhaust baffle plate, powerhead base gasket, or of course water in the fuel. The most common cause is the head gasket so I'd recommend you remove the head and start there. If no problem there, that's no lost cause as you need to clean the cylinder anyway. The second place to inspect, if the head gasket isn't the problem, would be the exhaust baffle plate area.

 

[BonairII]

Re: 1978 Johnson 70hp. Bad head gasket or exhaust gasket?

Yup, that was a typo. It's def a 70hp.

Is it possible to have a bad head gasket even though the compression numbers are so close? I would've figured a bad head gasket would've shown the bottom cyl compression a bit(or a lot) low?

 

[orbanp]

Re: 1978 Johnson 70hp. Bad head gasket or exhaust gasket?

I saved the following post from here, I am sorry but I do not know the author.
This is true for all three-cylinder motors from that era to some extent.
Peter


water in lower cylinder on johnson 65 horse

That motor is notorious for warping the inner exhaust cover plate. There are areas next to the exhaust ports that are not supported and they warp. Then they leak and spray water into the exhaust port. Replacing the gasket is folly---you have to replace the inner plate too.

I can almost promise you that is the problem. Almost.

 

[BonairII]

Re: 1978 Johnson 70hp. Bad head gasket or exhaust gasket?

I'm going to open the exhaust housing today. Is it safe to assume that I'm going to have to find another inner plate at a scrap yard?

 

[BonairII]

Re: 1978 Johnson 70hp. Bad head gasket or exhaust gasket?

I pulled the exhaust housing off. It was a huge pain in the butt because I had to loosen the lower shroud and drop the lower unit to move the shroud.

Here's a quick video of what I found inside. As you can see there are some wet spots on the gasket itself. Does this point to the exhaust gaskets being the culprit....OR is it possible that the head gasket could be bad?

http://www.youtube.com/watch?v=biPDcz088ec

 

[durban]

Re: 1978 Johnson 70hp. Bad head gasket or exhaust gasket?

I saved the following post from here, I am sorry but I do not know the author.
This is true for all three-cylinder motors from that era to some extent.
Peter


water in lower cylinder on johnson 65 horse

That motor is notorious for warping the inner exhaust cover plate. There are areas next to the exhaust ports that are not supported and they warp. Then they leak and spray water into the exhaust port. Replacing the gasket is folly---you have to replace the inner plate too.

I can almost promise you that is the problem. Almost.

if it is the exhaust manifold plate here is the answers what to do
http://forums.iboats.com/showthread.php?t=528873

 

[BonairII]

Re: 1978 Johnson 70hp. Bad head gasket or exhaust gasket?

Head and exhaust gaskets will be here tomorrow. Anyone know for sure what the torque specs are for the head and exhaust bolts? My manual states torque numbers for particular size bolts, but doesn't specify torque for heads/exhaust housing...so I want to be sure.

Do I need use a gasket sealer or can I mount them dry? Will the gasket likely say "this side up"(or something similar), so I know which way to install them?

 

[durban]

Re: 1978 Johnson 70hp. Bad head gasket or exhaust gasket?

i go according to the grade of bolts & nuts i buy new , for grade 5 1/4 inch thread , i use 6 ft-lbs , grade 8 ,9ft-lbs , grade L9 , 11ft-lbs on the top of the head of the bolts there are normally lines the more lines it has the higher the grade of bolt , the lines are normally in like a star sort of arrangement on the top of the head , for exhaust i use a high grade bolt

 

[BonairII]

Re: 1978 Johnson 70hp. Bad head gasket or exhaust gasket?

If I have to sand the heads(warpage)...what grade sandpaper should I use?

 

[durban]

Re: 1978 Johnson 70hp. Bad head gasket or exhaust gasket?

if you gonna sand it by hand you gonna make even a bigger mess than what you have presently ,k do it like this my pictures will show you .
http://forums.iboats.com/showthread.php?t=528873
http://forums.iboats.com/showthread.php?t=530630
http://forums.iboats.com/showthread.php?t=532243

 

[BonairII]

Re: 1978 Johnson 70hp. Bad head gasket or exhaust gasket?

I pulled the heads today. Cylinder walls look great. I cleaned all the old gasket material off...what a pain the arse that is! Can I/Should I use some gasket sealer when installing the new gasket? I don't have access to OMC gasket sealer, so is there an automotive sealer I can use?

I pulled apart the exhaust housing also. Looks great in there, no pitting or corrosion at all. Same question for these gaskets....should I use a gasket sealer for these gaskets?

Just read my manual....says lightly coat gasket with OMC sealer. That was case back in the '70s...not sure if new gaskets still need the sealer or not. (I bought all OEM gaskets btw)

 

[BonairII]

Re: 1978 Johnson 70hp. Bad head gasket or exhaust gasket?

Update: Not great news. Installed the head gasket....and lost 10lbs psi compression on all cylinders. Not sure if that would come up a bit after the motor runs for a bit.

No spark on bottom cylinder. Put a old spare coil on bottom cyl and still no spark. Swapped middle and bottom cylinder wire in the PP and bottom cyl sparked. Probably a bad PP, correct?

Worse case scenario, I now have a "parts motor" for my 75hp Evinrude.

 

[BonairII]

Re: 1978 Johnson 70hp. Bad head gasket or exhaust gasket?

This sound like a bad PP?

 

[BonairII]

Re: 1978 Johnson 70hp. Bad head gasket or exhaust gasket?

Anyone?

 

[durban]

Re: 1978 Johnson 70hp. Bad head gasket or exhaust gasket?

just a tip for those that don't know check the face of the block where the manifold plate goes on from corner to corner , especially from the slant angle on the face to the corner where the square is for the exhaust outlet i find that it warps in that area too i have found 2 blocks like this , get a good engineers rule & apply little pressure from 0 to 150mm & observe the the end do this on all 4 corners
http://forums.iboats.com/showthread.php?t=531964