Bored today, usually a bad reason to mess with stuff that doesn't need messing with.

502 Mag MPIs.

Decided to try some decarbon-ing of the top ends of my motors, based on the recent leakdown results that *might* have indicated some carbon contamination of exhaust valve sealing surfaces. Opinions are all over the place as far as using seafoam, ATF, water, MMO, etc, but most of what I read seemed to suggest that water would be most useful for exhaust valve deposits whereas the petroleum based products would be combusted and not really make it to the exhaust valve. I dunno.

Stbd motor went fine, fed a very slow trickle of water through the PCV line, got about a pint fed in over a couple minutes, shut er down, no problem.
Port motor, same thing, but after literally not even a quarter ounce of water fed in -- standard sprayer bottle is within a half inch of full!, the motor started knocking. Holy crap, WTF did I just do.

Here's a video. Looks like my boating season just came to a screeching (knocking) halt.

Oil pressure is steady at 60psi, no glitter visible on dipstick (yet).

Been thinking about getting out of boating altogether lately, this could be my sign to do just that.

Sounds like valve train to me. Marvel Mystery Oil works wonders for top end cleaning and lube when added to your fuel. I would not run that engine again until you do some exploratory surgery under the valve covers at least.

Are you sure that its not an internal exhaust flapper clanking??

It sure sounds a bit too loud for that in person, but I guess I can't say with certainty. I have Corsa tips with the silicone rubber tipped flappers.

The speed of it seems to point to top end, I think it's about half the speed of rod knocks I've heard before.

I used to have a freind that years ago would stop to help Harley guys broke down on the side of the road (this was shovel head days). He would work with them for a while then look at them and say "I know how to fix this!".

They would be looking at him like how?

He would say HONDA,,,,,, then jump on his bike and take off.

In his honor.

You ain't wrong.

At the beginning of the video its a irregular beat for a few revolutions then it gets consistent, where exactly did you pour in the water ?

Yeah, it does seem to be a little inconsistent at first startup, then it gets a little more regular or consistent.

I pulled the PCV valve out of the port valve cover, then fed water into the rubber PCV line which enters the underside of the plenum just after the throttle body.

Kinda sounds like an exhaust leak.