74 Johnson 50 low compression

[ronward]

I have a 74 Johnson 50ESL74M that I can't get to run long enough to adjust everything. I already know it has low compression on one cylinder, 90 lbs on one, 150 on the other. Could this be the cause of it not running good or should it still run with those numbers? I've been told it should still run but might be down on power a little bit. Please reply with any opinions, I'll gladly try anything at this point.

 

[Joe Reeves]

Re: 74 Johnson 50 low compression

Don't go any further until you remove the cylinder head in order to inspect the head, head gasket, piston, and cylinder walls. Hopefully your problem is simply due to to a blown head gasket.

 

[bonz_d]

Re: 74 Johnson 50 low compression

Sorry to say it but that much of a drop on one cylinder is serious. Most likely more than just some stuck rings. It should run but will never run well or smooth. Also by running it you may cause more damage.

I would suggest pulling the head off of it and looking at the cylinders for scoring and the head gasket for a blow out. You may get lucky and find it's only a blown head gasket in which case it would be a relatively cheap and easy repair.

 

[Chris1956]

Re: 74 Johnson 50 low compression

Gee, isn't that compression quite high for that motor? Sounds like the compression gauge is reading high. Did you take the compression test a couple of times to confirm the difference is consistant?

 

[ronward]

Re: 74 Johnson 50 low compression

Gee, isn't that compression quite high for that motor? Sounds like the compression gauge is reading high. Did you take the compression test a couple of times to confirm the difference is consistant?

I was thinking the same thing but I checked two other motors at the same time and they checked fine. Only this motor had crazy numbers. Thanks for the head gasket tip guys. I had my head wrapped around a stuck ring or a carb issue so I have the low cylinder soaking in seafoam and have cleaned the carbs real good...AGAIN. Next thing is I will definitely pulled the head. Thanks again

 

[bonz_d]

Re: 74 Johnson 50 low compression

Gee, isn't that compression quite high for that motor?

Well let's see, I've got a 1975 50hp that is pushin 150lbs on both, a 1983 60hp that has 140lbs on both and a new to me 1987 50hp that has 145lbs on both. So I would say NO not high.

 

[ronward]

Re: 74 Johnson 50 low compression

That's good to hear. I checked it several times thinking 150 was high, but also hoping 90 was a fluke. no such luck. I'll get that head off and post what I find

 

[Joe Reeves]

Re: 74 Johnson 50 low compression

Those older 2 & 3 cylinder 50hp 65hp, 70hp, etc normally have compression readings of roughly 135 psi.... or higher depending on what might be sprayed into them.

 

[ronward]

Re: 74 Johnson 50 low compression

Don't go any further until you remove the cylinder head in order to inspect the head, head gasket, piston, and cylinder walls. Hopefully your problem is simply due to to a blown head gasket.

You guys were exactly right...blown head gasket. Sure was scared, I've already been on craigslist looking for another motor. I can handle a couple gaskets and a fresh thermostat a lot better than another motor. Thank you Joe and bonz_d.

 

[ronward]

Re: 74 Johnson 50 low compression

This motor has a pressure relief valve located directly above the thermostat in between the outer cover and the head. When I opened mine up the spring was there...but no relief valve. By looking at the paint around the area it is obvious nobody has ever been inside this motor. So where could the relief valve be? Can they get brittle and break apart over years of heating and cooling? I didn't find any stray pieces of anything inside. I know OMC wouldn't have designed it to be there if it wasn't needed. But what exactly does it do and what could happen without it?

 

[Joe Reeves]

Re: 74 Johnson 50 low compression

I have no idea where that relief valve might be but check the parts manual for whatever should be there and correctly assemble it as needed.

Note that the cylinder head bolt torque is 18 to 20 foot pounds, to be tightened in the following sequence.

9....10
5.....6
1.....2
4.....3
8.....7

 

[ronward]

Re: 74 Johnson 50 low compression

I have no idea where that relief valve might be but check the parts manual for whatever should be there and correctly assemble it as needed.

Note that the cylinder head bolt torque is 18 to 20 foot pounds, to be tightened in the following sequence.

9....10
5.....6
1.....2
4.....3
8.....7

I think I figured it out. I found some type of plastic that had melted and stuck to the coils of the relief valve spring. At first I thought it was something the pump sucked in but now I believe it must be what is left of the valve. Thanks a bunch Joe

 

[Joe Reeves]

Re: 74 Johnson 50 low compression

I think I figured it out. I found some type of plastic that had melted and stuck to the coils of the relief valve spring. At first I thought it was something the pump sucked in but now I believe it must be what is left of the valve. Thanks a bunch Joe

Good catch there..... Meltdowns like that do happen from time to time.