I still think you need more timing at idle.

but first I would check the power valve.

Then you can do this without driving start turning the advance up even as high as 32 or more at 1000 rpm see what it really likes for idle advance. Later turn it back till you can recurve the dizzy if need be.

That's a big enough cam even in a 505 and big cams like lots of idle advance.

I take it you have re set the float bowels?