I have a 1997 Yamaha 150 2 stroke. I've rebuilt carbs, fuel pumps, replaced fuel lines and it ran like a new motor for 2 days...then heard rapid tapping noise at 4700 rpms. shut it down and limped back to the trailer. Motor definitely runs and sounds different. Still starts and idles. Checked plugs and one is black. I tried to see in through the spark plug hole at the piston but couldn't really see much... I've spent two months restoring this boat and now I'm sick about this. Any ideas on where to go from here?