AFR slowly richens at idle

Cam is [email protected], .652 gross lift. (Slightly reground Mini-Express mushroom). Four-speed, 3.91 gears, 26.7" tires.

The cam is not the problem here - the idle is manageable as I noted in the first paragraph of my post. No plans to replace it, thanks. :p

I will double-check the timing, but I don't see how that could be richening the mixture with decreasing vacuum signal. Also why would it idle for 10 seconds and then start richening without the RPM changing?
It's set up with one very light spring and one heavy spring with loop, so it cranks around 15 degrees for easy starting, idles at 27, and slowly increases with RPM to 35@4000 (there's a little more advance still to come, but I will not stand next to it and rev it any higher while holding a timing light close to the engine) :rolleyes:

The point of my post is that something CHANGED on Friday and I could use some help figuring out what it was that changed.