Next chapter in the Mercruiser saga, if anyone is interested!!

I think that I have traced the problem to a rusted out manifold water
jacket. There is a hole through to the end exhaust port, and I expect when
it gets hot it lets water in the adjacent inlet port.

Better start looking for a secondhand manifold. Nearly $500 on the
Mercruiser parts.com website, which usually means the same in pounds by the
time we are ripped off over here. Anyone got a siezed up 3 litre Mercruiser
sitting out in the shed?!!

Cheers, Barleycorn.