Mr. Rock Crusher,

as per volvo service bulletins

starting in 1984, the timing was slowly advanced in an attempt to find a lost pockets of Power. I remember it to top out at about .90 or .95 which is what you recorded. (Original was about .50 as you mentioned)

there is also a relation between fuel mileage and the adjustment but not linear...(the fuel mileage i.e. peaks at .75 or something and then decrease...just an example)

too bad the adjustment is such a pain to do, or else you could plot a graph of the relation between advance and mpg

diesel rattle at mostly combustion noises, amplified by heavier (that gas engine) engine parts...try some Redline oil diesel treatment(or Mobil 1 as fuel treatment) and attempt to adjust each injector to exact same pressure between them (if you want to play with the Dreaded-Tillman-Injector-Seals again!!)

Has the injector pump been calibrated lately?

hope this helps

Dr.JV