I thought I had this concept down way back when I was a wee lad but recently came across something that puzzled me.

Now, the only way that vacuum advance makes sense to me is if the port is "timed" so that it does not see vacuum signal at idle. Otherwise you have full vacuum advance at idle! Right?

I was working on a car last weekend that had a factory carburetor and only one vacuum port, which was NOT timed. It had full vacuum signal at idle and ramped down from there.

How does this work? Is it really designed to have all that extra advance at idle? Doesn't make sense to me.