Carb Tuning / Vacuum Problems?

I don't think my carb is tuned right and my engine isn't running as well as it seems like it should. I got a new Holley 670 cfm Street Avenger carb this winter and I'm using it on the stock spreadbore manifold with an adapter. When I first got the carb, it worked pretty well right out of the box. However, I didn't have much of an exhaust on then, and when I got a full x-pipe system put on, it didn't want to stay running anymore. I played with the idle mixture screws and they seem to work best a turn and a half out of the stock location, which seems like a lot, but that's what I got when I followed the instructions I had for adjusting them.

Here's my problem now: the car has very little vacuum at idle in drive (fluctuates between 2 and 5) and to get it to idle at ~800 rpm in drive, it wants to idle at about 1300 in park. I also pulled the air cleaner today and noticed black soot on one of the secondaries in the carb. It seems obvious to me that something isn't set right. First of all, the timing varies around at idle in drive but I know it's close to 12 degrees initial, and needs to be around 14. Other than that, I'm not sure. There is a pretty loud hissing noise that sounds like it could be a vacuum leak, I only really hear it at idle in drive (below 1000 rpm or so), but it's loud enough that I can hear it in the car.

Here's a video I took today. I start the car and have it in park, then I put it in drive and you can see how low and varying the vacuum is. I also got a little video of the black soot-like stuff in the carb. It has a Comp Cams 280H cam in it, so I know the vacuum won't be great at idle, but 2-5 seems way low even for that (around 700-800 rpm in the video, I think).

Anyone got ideas? Do I have a vacuum leak somewhere, carb need better tuning, other problem, all the above?

[ame="http://www.youtube.com/watch?v=byjEbGuzMwA"]YouTube- Duster Vacuum Gauge[/ame]