I have 1966 coupe with an all original 289 and an autolite 2100 carb. replaced wires, plugs, points, condenser, cap and rotor. Performed a tune up with vac line plugged. Gap is 0.017 and timing set to 6deg BTDC per manual. Verified that TDC is accurate by confirming top of stroke for #1 cylinder matches TDC mark on balancer and position of rotor is correct. Runs fine with vac line plugged. revs and returns to timing mark without any issues with vac line plugged. When i plug in the vac line and try to rev the motor it bawks really bad and nearly stalls. Tried changing timing to about 8deg BTDC per another's suggestion and no change. Tried drawing vacuum on the vac advance and it moved and held position until vac released. It was running fine a couple of weeks ago but now i'm afraid to take it out of the garage. Open to any advise. Regards.

 

does it have power brakes? Perhaps the booster is leaking.
You could try to find a vac leak using carb cleaner and spraying around suspect areas and listening for a change in engine rpm.