Nice little 66 Fastback. 289, auto car. Seldom used and had poor running issues. Older motor build and it really needs a valve job to be totally fixed.
Got the car and it was shaking all over and had numerous flat spots. Checked timing and it must have been near 22 degrees before top dead center. And it had an emission vacuum advance with just the forward hose hooked to ported vacuum. This is the second car that I have recently got with the back port not used. Back in the day people would plug the back line or just remove it to "de-smog" their cars.
What I have learned is these emission vacuum advances without the back port used will allow as much as 20 degrees of advance and it comes in near immediately. Way to fast and way to much. The car under a light load can ping extensively and even shake.
I installed an adjustable vacuum advance and shortened the curve to around 10 degrees for it alone. Spun the initial timing back to 12 degrees before TDC and adjusted the air screws. Engine purrs like a kitten under power. It has a weak cylinder on the right bank that shows up at idle, but we will address that this winter. In fact, the more the car is driven the better it seems to run. Maybe a chunk of carbon on a valve seat or a stuck/grumpy compression ring.
The use of the adjustable vacuum advance to shorten its curve couple with 10 to 12 degrees of initial advance seems to work so well on all of these basic stock V8's. I have had great luck with this set up.
Rob