Managed to get some time today, so ran it with the cover off again and it was pretty clear where the issue was - valve 3 had a huge gap. Could I have missed this one when setting the gaps after putting the head on? Certainly possible
But on setting it again I noticed that the spring seems a lot less stiff on this one - it can be compressed when adjusting the valve clearance (ie the rocker will push it down). This doesn't happen on the others so I'm thinking that spring needs replaced (mine only has one per valve). But I re-set the gap anyway as best I could and triple checked all the others. Upshot of all that.... the noise has gone! Result..
This is what it sounds like now:
Remembering that it used to sound like this:
So a huge difference.
It still sounds like one of the valves is a bit 'sticky' to me, but I'm going to see how it beds in for a bit. It may well be the dodgy spring so I'll get that changed when I can.
'course now you're all thinking 'did he strip the entire head and disassemble everything when all he needed to do was check the tappet gap?' - well i'm asking myself that too...
But i'm pretty certain the gaps were all good before i stripped it as it was the first thing i checked. That's my story anyway and I'm sticking to it!
And the stripping down was actually fun - makes a huge difference when it's not your daily drive any longer so no time pressure to get it back on the road, no more working on it in the dark and rain.
Cheers for the company while i worked through it - all your tips on what to check and how were incredibly useful as always.
TS