Street Demon tuning experience

For the AFR reading, somewhere around 12 at cruise and 15 at part throttle acceleration is what I wound up with using the smallest jet and biggest rod.

I just adjusted timing. When I went to 12 the idle mixture screws had no affect even at 600 RPM. I ran the screws all the way in and the car still idled normally.

At 35 degrees the car almost shuts off running a screw all the way in, starting around 700 RPM. With both screws in it shuts off, especially if I reduce the idle speed. There is about a 100 RPM increase going from 20 degrees to 35 degrees. It starts to decrease again around 40 degrees. I left it at 35 degrees timing and 700 RPM with screws about half a turn out.

I also found my throttle doesn't always return to base idle. It goes back to 720 instead of 700 sometimes. I noticed this when backing the screw out; it wouldn't back down until I pushed the throttle and then the car would stall. So I moved the throttle return spring around and the best I could get was the 20 RPM change, but I figure that's close enough.

So if I leave my timing here then I have to lock my distributor, which seems weird to me. I just don't have a concept of why it would need this much initial timing. I have a firecore distributor that's adustable so it should be easy enough to set to 0 advance. I'll have to hook up the O2 again tomorrow and see what it does.

I also put in a fuel pressure gauge and the fuel pressure is around 5 - 5.5 psig which is within the range the booklet says for the carburetor, so that shouldn't be an issue.
Sounds like you need to verify TDC, if it's running good at 35 degrees at idle something is off. You did find the issue though, not enough timing at idle.

I had it kinda backwards with the transfer slot situation, I assumed it was somehow pulling too much vacuum and thus fuel mixture at idle, whoops...