Vernalift - water muffler - gone bad?

[hglucas]

I have a 20hp Yanmar on my Morgan. I am getting back pressure between the exhaust manifold and the Vernalift muffler enough that it blew a hole in the hose. I thought maybe the muffler was bad but I ran a water hose in the Vernalift muffler and seems to flow good.
This only happens when I run the engine at high RPMs. If I idle everything is OK.
Do Vernalift mufflers go bad? Could the raw water pump be bad?
Any one have any ideas what might be causing this back pressure?

[NorthUp]

Could be that a section of exhaust hose has started to delaminate internally,
open both ends and shine a torch through.

[captbillc]

i would also check the hose between the muffler & the overboard discharge & look inside the muffler. something way be loose or a piece of hose is causing restriction in the muffler.

[r.furborough]

Yanmar exhaust elbows are notorious for getting blocked, remove it and clean it out. It will eventually cause your engine to over heat.