90hp Mariner Knock

[Duckhunter71]

Picked up a used 90hp Mariner. It is from the early 90s (OD163,,,).

Compression is good and even across all three cylinders

Starts up good,
Runs great on Muffs including in gear.

Mounted it on a boat, took it to the ramp to do a on the trailer test.

Started, pumped water, shifted fine.

Went two-thirds throttle in the water. got a knock. It is a dull knock, not one that sounds like a rod is going into orbit.

With the compression even and good, is it a bearing problem?

What do I need to focus on before the tear down.....

 

[Faztbullet]

Re: 90hp Mariner Knock

Check lower unit 1st for large metal debris ,if ok it could be a center main or lower bearing or a cracked piston skirt will cause a knocking noise(piston slap)

 

[arimatation]

Re: 90hp Mariner Knock

I have a 1994 60hp Merc I bought and heard a knock when I ran it at home. I was thinking bearing. But I started it another time and the noise seemed to be a lot less than the first time.

I had sprayed the cylinders with Amsoil Power Foam that dissolves carbon. Then my thought was maybe the last owner did not do de carboning on the motor. I will be checking into this further.

Don't know if that is the problem with you, but I would think built up carbon would be more common than bad bearings.