burnt piston what caused this?

[danteknik]

hey there.
the motor is a 225 hp excel year 1989 but powerhead should be around 7 years.

Please have a look at the pictures. i noticed last time i was out that one motor was shaking not much but more that the other and blowing alot more water out the upper exhaust relief ports. did a compression test and found this sad news..
normally a burnt piston is caused by lean running or ? i did overhaul all carbs 2 years ago and probably ran the motors 30 hours since.could something else have caused this ? water in fuel,bad check valves,bad intake blades,leaking intake manifold ect..
I did comp test on all cyl and here is what i found
1=100 psi
2=0
3=90
4=90
5=87
6=88

what does a clever man do now ? redo all of them with new piston and rings
or just new piston i 4 and rings in the other or just fix 4.
keep in mind here in europe it is hard to come by motors this size and just the shipment of an exchange head to the US and back is a killer.. does any one know a good european contact or ?

thanks

[boats.net]

Looks like detonnation, could be lean condition(carbs), poor fuel, lugging engine(not turning enough rpm's), timing, or overheating.
Have cylinder honed and checked to see all other cylinders are ok, rebore as needed.