Ashtray, I have a vague recollection of the switch being there but not connected up and its vacuum ports were damaged. I might be recalling a different car though. Take a look for it at near the upper radiator hose on the manifold and let us know what you see.
I finally found a clearer diagram of the vacuum lines. Imagine it being here
My thought is that if vacuum is being applied to the retard side of the distributor, you won't get the advance timing you need. Setting static timing by the manual won't take wrong vacuum connections into account. Somewhere along the way, you may have set per the book but the retard side vacuum throws that setting out the window.
In any case, I think Amervo is hot on the trail. One way or another, there is a lack of timing advance. This is very hard to check because you can't watch the timing mark while you drive.
