40hp strange sound, knocking.

[TD40]

I was running my 1987 40hp Yamaha 2 stroke close to WOT and then it started a loud tapping or knocking sound. It sounded like a bad valve on a 4 stroke motor. It vaired with RPM. I shut the motor down a called for a tow. When I got home I ran the motor on the hose and did not hear it. I checked compression and it was pretty good. It was 90, 90 and 89. I have ran it a couple more time trying to find the noise and have heard it again but not as bad. I have not taken it above idle since the first trouble. I am pretty sure it is internal maybe a rod, main bearing or bad piston rig. Anyone have a suggestion? I am thinking about disassembling the power heard to see what I can find. Looking for tips before I go in.

Motor runs fine other than the tapping sound.

Thanks for any help.

[boats.net]

Remove the cylinder head and check condition of pistons and cylinder wall. Check for loose play in piston's by bringing to top dead center then push on piston while holding flywheel from turning, should not be any loose motion.

[TD40]

Thanks I will give that a try.