I suspect if a valve spring was giving problems on an 80 diesel you would have had the valve hit the piston. On the 24 VALVE 80 there is a minute valve overlap of only a few thousandths of an inch on TDC overlap.
I take your point Frank, but I wouldn't rule it out. I'm also not advocating Rich taking the head off yet to investigate the condition of the valve seats, but a prod around the springs with a screwdriver checking their integrity would only cost a cover gasket, and a couple of hours' fettling.
It seems he's getting no closer to the problem, and for sure its not nothing, so it must be something.
In my case, and I don't remember what engine it was, the valve operated correctly with the inner spring broken, it just didn't make a good seal which allowed some compression escape, enough to cause a lumpy idle. It appeared to drive ok.
I have no idea if the variance between 480 and 500 psi is significant, on the readings, but if it is, it could be worth starting with a grind of the valve seats, it's certainly cheaper than the alternative probable causes.
Its only 4% down in the extreme so I'd imagine the readings are even enough, unless they're all down which is unlikely. Is the firing psi a published figure, does alone know?
A quick edit edit here after doing a bit of gluegleing... and I'm only quoting what I found
Standard 498psi (manual says 500psi);
minimal acceptable 360psi;
Compression Ratio 18.6:1
One guy on ih8mud got readings of 450 psi on 5 and one at 430.
So I recon Rich's readings are good and even (regular) enough to rule out a bad valve seating...