hello dear friends.. i need help...

i have a 2000 fountain lightning with 500 hp's.
i replaced the manifolds 3 weeks ago and while there,decided to get the heads off and send them to get checked and gone through..boat has 90 hours....salt water used...

they came back telling me that the valves were needed to get sone as they were pitted... went to check it out and yes they were... they needed to clean some and replace a few and re-do some seats..

put all back in the boat and went out last week...great run !
this week,great run...on the way back into the marina,i was at idle going to my spot and one motor stalled as i was looking in the engine bay ! my firnd was at the controls ..at idle ..... i was looking for any signs of water leaks etc...all was fine...i figured it stalled as it was running a little low at idle..
i restarted it and went back to what i was doing...stalled again !!!! i started it again and as i was going back, i heard this all fo a sudden loud knocking sound..in one shot ! knock,knock,knock ...oh my god...shut the motor off !

too compression test and one cylinder has zero..a few 80-80-150....
took off both heads that same afternoon.

the seats have broken like glass !they are spread in a few cylinders..poked into the piston surface....on a few pistons....did some damage to the head surfaces as well...

how in hell can all these seats desintegrate like this ????

that machine shop are really good usually..they do lambos for me ,ferraris heads and blocks...

did they use bad quality seats ? were the valves badly seated and broke the seats ? was the mixture bad and hurt the seats ? too lean,rich ?hell, i am lost here and cannot understand how ?
they are closed for another 13 days...so i am trying to find the cause....

the other motor sounds fine... i will yank the heads off as well...wont take a chance and hopefully, there is no damage there..will take compression monday...but still, wont take a chance these seats break and might as well get them checked or changed now...

I had this happen to me before.It took me several rebuilds to figure it out.What I found was a lean condition which caused the heads to get so hot the seats fell out on a few holes.Once I fixed that motor,the other would do the same.I did both motors 3x each until I figured out my problem.My problem was low charging voltage at high rmps.When wide open,the electric fuel pumps werent getting a full 12+ volts,only 9.So my fuel pressure was low when it needed to be high and created a lean condition!
Another boat I had did something similar.One of the float valves in the carb stuck shut which caused the engine to run on only half the carb and with a singal plain manifold this created a lean condition.The seats didnt come loose but it did tulup the valves because of the heat! hope this helps.

Reversion can cause this problem. Dripping cold water on hot seats and valves is never a good thing. Can your engines be reverting? Is the idle too low? Too low of an idle can cause the engine to revert water.

i never let of the throttle...when i slow down...as a precausion..even though i have flaps on the pipes...and there was no water in the cylinders when i took out the plugs...

one thing i can think about is that motor was running rich...

I was looking at buying a pair of 500hp's a few years back that had aluminum heads on them. The reason the heads were replaced was because the original iron heads kept dropping the valve seats. He said no matter what they did they could'nt keep the new seats in the heads. If it were me I would give some serious thought to replacing the heads, might save you some grief in the long run.

that doesnt make sense....once valve seats are well installed, they never fall off !@

Did you replace the original valve seats?

i was told they replaced a few .... now they may have used cheap quality ? or non marine ?
now that motor was running rich ,...can that destroy seats ?

i will check the other motor tomorow...that motor was not running rich ..lets see if there is any damage or lost compression due to premature damage of those seats...

Did they magnaflux your heads, are you sure there are no hairline cracks, if it's a reputable shop I doubt they put all the seats that fell out wrong.

Which manifolds did you replace????? exhaust manifolds or intakes??????

The valves most likely became pitted beacuse of cracked Gil manifolds leaking water.

Also, there are 2 kinds of reversion. The water that gets forced up the exhaust from a wave is one. The other type is caused by cam overlap causing water get sucked back up idle. The 500hp's had some issues with reversion at idle and changed risers because of it. The lower the idle rpm, the easier it is to suck water back up the risers.