I had the same problem with my 460 when I hooked it up to the hose. There is away to flush the sand out of the block, on my block they looked like wing nuts not sure if they all look like that, there is one on each side below the heads around where the freeze plugs are. I opened both of them and ran the water threw, I was shocked how much sand came out even had to poke the holes a few times to unclog the opening. The problem went away after this, maybe this could be the issue on your setup. I’ve since heard that lower Blythe where I tend to go is notorious for clogging blocks and ruining wear rings because the fine sand that gets turned up in the shallow water. I now flush my motor each year when it’s put away for the winter.


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Check your injection lines at the entry to the header also they will corrode and plug up also

1977 Challenger

When I bought my boat with a 460, I had a similar problem. Water ran out of the dump and only one side at idle, then both sides with some rpms. After a ton of reading I found this diagram on a thread deep in here (thanks to the original poster), and it solved my problem. The last "plumber" attached the dump at the same level as one side exhaust and the other side was connected at the top of the water manifold/tree. Maybe this will help. Good luck.