Need help with 25 Mercury noise

[Lakeside Dude]

Hello, first post , My son bought a boat in Feb this year with a 1987 Mercury 25 hp. that purred like a kitten up until a few days ago.
We found a fouled plug and got it cleaned up and got it running but it makes a terrible noise like nuts are loose inside the top end.
Any idea what might cause this ?

 

[johnnygjr]

serial # needed to be known,elec start,pull start???

 

[racerone]

Do a compression test.--------Perhaps remove bypass covers on the cylinder that had fouled plug.---------Inspect piston / rings.

 

[Lakeside Dude]

serial # needed to be known,elec start,pull start???

Electric start

 

[fastjeff]

What oil mixture were you using?

Jeff