Well, that explains it then. This is the view peeking down the inlet to the cat. The cat is not meant to look like that jumbled pile of bits and pieces.
I got up this morning having decided that the problem was either a) something to do with engine management or b) a blocked exhaust. As you can see it was the latter. This blockage would have reduced the flow of gas through the engine which effectively is what was causing the rich mixture.
What isn’t so clear, of course, is what to do about it. In particular, why did the cat fail like this? I could, of course, just weld a new cat into the exhaust but how can I tell if this just happens again.
To compound the whole thing, here’s another view, in this case looking up the silencer.
This whole thing means that a failure in a rather expensive component messed up my whole weekend. I was already angry enough about the whole idiotic cat business (reducing CO emissions in places like Snetterton is pointless; doing so in such a way that heavy metals are sprayed all over Norfolk is even more daft) but this really takes the biscuit…