Ok, here's the deal. Both motors 300 hrs. 454's, Edlebrock marine aluminum heads, carbarated, 502 roller cams, EMI thunder exhaust.

Port motor started to run a little rough. While doing a leak test, all holes had less then 10% leak. I'm going WOW great, un-till I get to #5, 100% leak... oh oh.

Pull the head and found the exhaust valve had lost 1/2 the head. Yep, broke half the head off, amazing part, just one small mark on the piston where the broken valve piece hit the piston and shot right back out past the broken valve and into Lake Havasu. Pretty lucky it didn't beat up the piston, head and more. (I did buy a lotto ticket right away, but looks like I used up all my luck on the valve, s#*!)

Anyway, wondering way it broke and looking over things, the valve spring was broken and there was pitting on the stem side of the exhaust valve head. Also, the remains of the valve has corrosion and the seat is corroed.

So I check the other motor, and #5 has 90% leak and #3 has 35%. All the other holes are less then 10%.

So now I pull that head and find the same thing, but no broken valve. Corrosion on the exhaust valves and seats.

It looks like a small amount of water is getting to these valves. Just #5 and #3 exhaust valves on both motors.

These motors don't have big cams with lots of overlap. The exhaust system is EMI Thunder with 4" risers.
The risers point down to the tramson tips, so I see no way of water running back into the exhust manifolds.

I did pressure check the mainfolds and found no leaks. I have not been able to pressure check the risers, but looking very close with a bright light I found nothing that looked bad.

Has anyone else experienced this? Could it be condensation building up in the manifolds when I shut off the motors and running down the inside of the manifolds? If so, why just the 1,3,5,7 side of both motors?
If it was condensation, you would think it would happen on both sides of the motor. Same thing if it was reverision, why not both sides?

Any ideas are welcome. I'd like to know why this happened so I don't go through this again.

Thanks for the help.

Post the cam specs. Do you have the short or long risers????
It sounds like reversion to me.

I agree reversion or a water leak. Some time you can not get them to leak until they get hot.

I bought the boat used and don't know the cam specs. Was told they are Clay Smith Cams and are like a 502 grind, for what its worth.

Not sure how to measure the risers, but they are 16" from the center of the flange to the end of the tube.

If it is reversion, why not all cylinders?

and if it is reversion, how do I stop it, short of changing cams?

thanks again.

Those sound like the longer risers.
There is no standard 502 cam grind. There are hundreds of different ones and they are interchangeable with all big blocks.
It could also be cracked exhaust manifolds that are leaking slightly. I had the same type pitting of on an exhaust valve due to a cracked Gil manifold.

I would inspect the insides of the exhaust manifolds very closely for hairline cracks.

Not sure why it would be the same on both engines. Could just be a coincidence or the manifolds cracked in the same place.
If its actual reversion, then your options are to change cams or change exhaust.

I have seen in the past when a boat sits over winter with a certain cylinder's valve hangine open and you dont fog the engine it will rust the valve and seat,espically around salt water..

I'm taking the manifolds and risers to Eddie Marine to be checked. I did pressure test the manifolds but was not able to test the risers so I'll let them do all for me.

Thanks again for the help.