1973 Johnson motor question.Low compression on one cylinder?

Question

I have a 1973 johnson 50hp on the top cylinder I have 120 pounds and the bottom I have 90 pounds.What is the most common reason for this? I noticed in between the cylinders on the outside of the head there is a rust stain on it where the head bolts to the block. and on the front of the head there is another cover I think it covers where the water runs through the engine on the bottom of this plate there is a spring and some kind of valve well all around the spring there is some kind of red glue or apoxy that has the spring all the way covered is this suppose to be there. If you can help please do so and thank you in advance

Answer 1

everything the 80' s kid said was correct. When the engine is running do you feel any leaks around the side of the head. Your correct too, the top cover is for the water circulation. The red epoxy with the spring on the top cover on the top of your engine is the thermostat housing, you should have a thermostat in there too. the epoxy is probably because they do not have a gasket- which i doubt because they are so cheap, more than likely there is a nick or uneven surface, and they put the epoxy to stop the water form leaking. You could try removing the head and checking the condition and replacing the gasket and retorxing all the bolts, and check the compression again if this doesn't work you will need to rebuild the engine..good luck

Answer 2

on an older omc it is probably the head gasket

Answer 3

Cylinders can lose compression for many reasons, but in the end - something is letting too much air pressure out.

Leak in a head gasket, worn o-ring on piston, leaky intake or exhaust valve, warped cylinder walls, even a spark plug not sealing right.  The rust stin ont he head bolts may or may not be anything to worry about, but that bolt hole, and the gasket around the bolt head all need to be sealed tight.

Not sure what you mean with the spring and red glue?  Are you sure its not some kind of red grease?  Glue and/or epoxy would not last very long in that spot, so I have no idea what that could be.

Honestly though, a semi-complete rebuild is usually one of the only ways to restore the compression, I would learn to live with this for now, as long as the engine is working good enough.