Bit of an update. Haven't been on LPG for quite some time now but the noise has persisted and it has gotten to the point of getting on my nerves. Started tearing into it to diagnose further.
First off came the exhaust header on bank 1, with the hope it was a visible crack in the header, or a gasket blow. Neither appear to be the case. There was a small mark on the inside of the heat shield that might have been causing it as it heated up and rubbed against the header pipe - but that seems too easy.
Next off came the upper inlet and rocker cover on the same side to inspect the rocker gear etc. Nothing unsual there, so off came the lower inlet and valley gasket. As the whole side of the engine and rear have been soaked in oil - it was all justified to reseal everything at the very least.
While staring into the abyss I'd gotten myself into, I couldn't help but notice the cam wasn't exactly as pointy as it could be...
As it now stands...
The best lobes:
The rest of them...
And the lifters:
Second up from bottom right in the picture is actually #8 errr inlet? I think. It has been hammered - and the top of the lifter was dry when the valley gasket first came off, so I don't know if it has a blockage inside or what - but its definitely buggered.
Obviously all this lot is going in the bin. Should explain the engine feeling a lot less interested in getting things moving these days.
Next up is the ever-interesting compression figures. Once the starter motor had been stripped and cleaned of oil (was causing an arc in the solenoid that made an audible pop and burning smell most times you started), it went back on before taking all the valve gear to bits to test.
Bearing in mind cold engine with 'wide open throttle' (aka: whole inlet missing), all plugs out bar the tester, I got 170-180psi on all cylinders except #4 and #8, which were 210-220. This is the same kind of figures I had a couple of years ago on a cold test, so nothing appears to have changed here, but I'm still perplexed by those two weird cylinders. I've tested my gauge on a nitrogen pressure testing rig and it agrees with that - so I don't think the gauge is over-reading etc.
Inspecting the top of the pistons with a camera, #8 is pretty much clean, #4 has some build up - but if it was build up on the piston surface, #8 is clean yet reads high. So... I have no idea. As I say - it has been like this for ages, and doesn't seem to have affected anything that I know of.
About to order parts to replace the cam and lifters, a variety of gaskets, timing chain and sprockets, fluids etc. While I'm at it, the crappy stainless exhaust is all going in the bin too. Back to standard rear silencers, a set of cats up front, and a straight pipe in place of the middle silencer as I used to have. Can't have it too quiet :)
All in all... hopefully the noise doesn't rear its head again. I am going to borrow a crack checking dye kit to look over the exhaust manifold and see if anything crops up there. It isn't the hardest thing to change if it does still make noise down the line.
Lastly - the LPG is going for now. The injector pipes were all loose and the original petrol wiring loom has become crispy and is in need of repair before the inlet goes back together. I haven't used LPG in months, and the nozzle bungs will all just screw back in as and when I re-fit it.