That looks like a 496 head. Was it running a stock cam? I would not expect a stock Merc engine like that to get reversion due to cam timing, and the stock 496 exhaust manifolds have eliminated the wet riser joint that plagued earlier Merc big blocks. What was the cause of this?

Yes it is off a 496. Motor is stock with a whipple stage 2 tune and lightning headers. Im thinking the reversion was caused by the headers. They weren't the right fit for the boat. I had the heads redone and have installed dana marine headers which fit much better.. I also found out I had forty something hours of idle time on the motor and was told that could have contributed to it.

Reversion is caused from a Cam / Exhaust mismatch. Stock engines have small cams and should not have a reversion issue. Lake water coming in through the back of the boat into the engine is not reversion. Flappers should prevent this from happening.

My boat has flappers on it and the tips sit well enough above the water line. My issue was the downward angle of the exhaust towards the tips. It wasn't enough which caused water to ride back up the exhaust and into the four rear cylinders. The dana marine headers have a much higher rise which ads more downward angle.

Just to give you an idea of reversion with a big cam 262 276 @ .050 114 lsa. I bought new stainless marine gen iii manifolds and risers. Got a great deal on used tail pipes. The tail pipes had mufflers bolted on at one time. Now the stainless marine stuff is totally dry and uses dumps but I had 1 1/4" hole in the tails at the very end of the tail. The tails have a really good slope to them. If you stand back with a flash light and look in the tail while the boat was idling you could see a trail of water going the whole way up and over the slope. I never had water in the oil cause I'm aware of reversion from a previous build and fixed it immediately but I was amazed that it could actually pull water back that far...

Exhaust hose actually looks like it runs uphill to the tips with the Lightnings.

Lol, yes it does. They were just the wrong headers for the boat. Im really liking the Dana Marine headers. Quality is excellent and Dana was very helpful getting things right.

http://youtu.be/yOzeRLGLHQc First startup after freshened up heads and new Dana Marine exhaust.