If your shift timing is that far off after adjustment I would guess your bowden cable needs to be replaced/repaired etc. I dont think vacuum problems can cause that type of shift timing error...

Disconnect the vacuum from the modulator temporarily, does that change anything? it will shift very hard, but does the timing change or improve?

Hopefully more qualified folk will respond.

__________________
-------------------------------
'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car