Not sure how it got that way. Had the headgasket replaced some time ago. Maybe they did not time it correctly. Had someone help me do some power steering hose/pump repairs not that long ago. He was working in the vicinity of the distributor. Maybe he grabbed and turned it.
I don't have a specific timing procedure outlined in the Chilton's manual I have so I just hooked up the light and checked it. It was about 17 degrees advanced. 3 degrees advanced is the spec. It runs fine now with no pinging. In fact it seems to run much better with a lot more power. I should have checked the timing sooner but stupidly, I thought the ECU was controlling it since it has a knock sensor on the engine. But that must only function correctly once the timing is manually set correctly.
My wife drives it regularly and I only drive it occasionally.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13