Today was the first chance I've had to get on the water since I got the boat back from the mechanic (valve job).

Sunny, warm and flat seas ..... just a perfect day offshore!
Engine performed flawlessly for about five miles (58mph @ 4600rpm) then throttled back to enjoy the view and get ready to push the stick to the max. Got back on plane and noticed the engine running rough, peaked at about 4000rpm, and when throttling back to listen, I could hear a thumping "knocking" sound from the engine. I thing it was coming from the port side (near the batteries) but can't tell for sure.

I made a video of the engine (listen to the audio) as we came back in. The engine did make it back to port OK but you could feel the roughness, even at idle.

www.desertcom.com/videos/700rpm.WMV
www.desertcom.com/videos/1500rpm.WMV

Any idea of what happened?

sounds like something up top let loose...of left loose by the mechanic....hard to tell though....

Sounds like a really loose rocker arm. Pull the covers and look.

It sounds a lot like an engine I had sounded when one of those self locking nuts backed off from the rocker arm stud. If it is, replace all the nuts. Self locking nuts only have so many times that the self locking works.

or arms..

Hope it's not a spun bearing on the crank.

would a bearing have that chatter??

loose rocker

I would not run it anymore, pull the valve covers, turn it over with a breaker and find the loose rocker, once found make sure the whole deal is good, ftom the valve to the nut, maybe the valve failed but it sure sounds like a major knock from above. Also document the hell out of it or take it directly to the dude and say WTF.

That could certainly be a spun bearing. I had a 454 rebuilt 2yrs ago, the builder put a bearing in backwards = no oil = nearly identical sound after about 30mins.

Good Luck to you, hopefully its something a little less complicated