LA3

454 magnum. New Merlin heads. New risers. New Manifolds. Put boat in and was running about 4000 rpms. Lost power. Sounded like a miss. Then heard loud rattling noise. Engine would not allow me to go above 500 rpms. And had intermittent "rattle or banging" on port side. Sounded terrible. Looked like there as a white-creamy stuff in top of valve covers. Someone please help.
Boat is a Velocity. Thanks.

dyno

Start with the easy stuff and work your way up....electrical first, Timing, wires and cap.

dyno

new cam too????if so did you break it in?????

mopower

For starters , that sounds like a load of water. Could be a leaky head gasket or intake by the water passage. With new manifolds and risers it could also be reversion (if they are a different type). In any case the loud rattling DOESN'T sound good

aero-offshore

I take it you pulled the valve cover ,so not a dropped valve or broken rocker, creamy white stuff is water or moisture, knocking is probably detonation.I would pull the spark plugs from the side that was banging and check those cylinders for water, even run water through the hoses in that manifold to see if the riser gasket is right. Did it smoke out the exhuast, on the side with the banging? if it did, probably head gasket not installed right or not the correct gasket, if the heads are aluminum they need special ones. Dyno's right start with egnition first.