Advice on a 625 CFM STREET DEMON CARBURETOR for 273 commando

I ran (and still have) a Holley remanufactured 625 Street Demon for my Mustang 289. I wasn't able to tune out a stumble when the secondary air door opened (I followed all the instructions for tuning it) and if I tightened the door too much, it would go dead rich before the air door opened.

I will say, it had a lot of dried gas in it when I bought it, and the accelerator pump wouldn't even work until I had it on the car for 15 miles or so (I'm guessing it dried out from shelf time). I'd buy a new one. With a little accelerator pump tuning I might have gotten the thing working well, but I decided to try something else. I have a Holley 570 on it right now that's working pretty well for me, but I had to do a lot of tuning on that one to make it right too. I've rarely been lucky enough to have a plug and play carb... :)