First, with the car warmed up, remove the air cleaner and look down the carb. if the choke flap is mostly closed your choke isn't adjusted properly. There are probably directions on Edelbrocks website.
To check for vacuum leaks I would inspect all of your rubber lines for age, cracks when you bend them, etc. I replaced ALL of my rubber lines when I bought the car, most are 40+ yr. old rubber. If you have an auto trans
. there is one at the rear of the intake and one at the end of the trans
. also that rarely gets changed.
You may be able to hear a vacuum leak if the engines not too loud. If you spray ether around the intake and the RPM's pick up you've got a leak.
You can test the vac. advance to the dist. by removing the dist. cap, disconnecting the rubber hose from the carb. (not running), look inside the dist. at the arm that comes out of the vac. adv. and suck on the hose. The arm should get pulled into the vac. adv. a few mm (you'll see it move). If it doesn't your vac. adv. is bad (diaphragm on the inside dries out). You can get a replacement at the parts store <$20 or from a mustang parts vendor. Not too hard to change with a small needle nose and a pencil magnet so you don't loose the C clip.
'67 Coupe 289 stroked to 333, AOD, 9" w/3.50:1, PDB, Candyapple red w/red deluxe int., PS, ext. decor group, light group, foxbody seats.
'09 Black Warriors In Pink with glass roof - the wifes Mustang.
"If it ain't broke, I haven't fixed it yet" -Jon