1981 Johnson 70 hp in good condition
500 hours (? 4th owner)
Saltwater use, but well cared for.
Blown head gasket last year on bottom cylinder with saltwater intrusion. I replaced the head gasket. Compression numbers of 120-120-105 afterwards. Ran it some since then and compression has dropped to the 90-95 range in the bottom cylinder. Still runs good. Starts and idles okay.
I just ordered a new boat because the hull is cracked on my old rig. I’m using this old motor to power the new boat. In the meantime I’m getting it tuned up. I like tinkering with these old OMCs, and I don’t mind spending money on it to keep it running.
I pulled the head again and inspected the cylinder walls. There’s some very light scoring in all cylinders, but nothing catastrophic. No metal shavings. The top cylinder head and piston has some dings from a past event. Bottom piston is slightly more loose side to side than the other pistons. I couldn’t tell much about the rings from that view. Edges of the ports seemed fine and the engine turns smooth and free when rotating flywheel. There is a place on the cylinder head at the sight of the bottom blown gasket that looks a little suspicious - slightly low and pitted. Other than that every looks decent.
Is that compression difference worrisome and is it okay to slap it on the new boat and go? Or should I tear it down and and completely overhaul it? Considering replacing the cyl head and seeing if that helps. I did a Seafoam decarb with multiple rounds and it’s still smoking so may also need more decarb. Any help is much appreciated!
Thanks,
JW
500 hours (? 4th owner)
Saltwater use, but well cared for.
Blown head gasket last year on bottom cylinder with saltwater intrusion. I replaced the head gasket. Compression numbers of 120-120-105 afterwards. Ran it some since then and compression has dropped to the 90-95 range in the bottom cylinder. Still runs good. Starts and idles okay.
I just ordered a new boat because the hull is cracked on my old rig. I’m using this old motor to power the new boat. In the meantime I’m getting it tuned up. I like tinkering with these old OMCs, and I don’t mind spending money on it to keep it running.
I pulled the head again and inspected the cylinder walls. There’s some very light scoring in all cylinders, but nothing catastrophic. No metal shavings. The top cylinder head and piston has some dings from a past event. Bottom piston is slightly more loose side to side than the other pistons. I couldn’t tell much about the rings from that view. Edges of the ports seemed fine and the engine turns smooth and free when rotating flywheel. There is a place on the cylinder head at the sight of the bottom blown gasket that looks a little suspicious - slightly low and pitted. Other than that every looks decent.
Is that compression difference worrisome and is it okay to slap it on the new boat and go? Or should I tear it down and and completely overhaul it? Considering replacing the cyl head and seeing if that helps. I did a Seafoam decarb with multiple rounds and it’s still smoking so may also need more decarb. Any help is much appreciated!
Thanks,
JW
