still on the trail of the elusive fault...
fitting new leads and posh red coil made it run significantly better. However, the basic fault is still around...
looking today I narrowed the rough idle down to an erratic #1 cylinder by the brilliant tactic of unplugging plug leads one at a time. I'd not bothered with this before as the fault doesn't present as a regular misfire, sometimes all 8 are firing, sometimes not.
So, out with the compression tester, crank 'er over, 4 bar. Bugger.
now being as it's just had new rings in honed bores, I'm inclined to suspect the head or head gasket. Shouldn't be the latter, the surfaces were clean and looked good when it was reassembled, and it was torqued down carefully.
I admit that when putting it back together I didn't actually examine the heads in detail, nor did I grind in the valves - I did examine them reasonably closely for evidence of burnt exhaust valves or recessed seats, and saw no such evidence - I figured therefore that I'd leave it well alone
- I didn't at that time suspect a head fault; if you've just found a damned great hole in #6 piston, it's not hard to work out that that was the piston which was misfiring...
So, head off for a looksee, probably at half-term in a month or so, and hope that it's nothing too serious. It's not losing any coolant, which is hopefully a good sign.
The thing I don't understand is how this fault affects the running so impressively. The only thing that spring to mind is that it could be a dodgy inlet valve, and thus be blowing back into the inlet manifold, and so upsetting the gas supply at low revs. At high throttle openings and higher revs, there's lots of air coming into the manifold, so a minor leak back wouldn't show. Now a dodgy inlet valve is an unlikely beast, really, but it could be.
Is it possible for an hydraulic followers to be to "strong", thus not allowing the valve to shut properly?
I guess this is the point at which I pull the cam and put the other cam, followers and pushrods in, too - would make sense, and eliminate any possible faulty follower, and maybe the other set of rocker shafts, which are nearly new, to eliminate a sticky rocker. Didn't notice any problems with the rockers when reassembling recently, though.
I now suspect that this fault has been around since before the piston failure on #6, and was masked by that, and didn't get fixed at the time. The engine showed signs of a weak cylinder, but I'd assumed that this was #6 on its way out, evidently wrongly.
Just hope it's not a cracked head...