i had same problem, found a large vacuum hose loose at rear of carb,it would start, but had to keep screw on carb all the way down or hold it at about 2000 rpm to keep it running, after i fixed vacuum leak it idled down at about 800 rpm with no trouble, have you got gas tank on car are running on a fuel cell in the trunk, if you have a manual fuel pump and a fuel cell that gravity flows to your pump, it may be letting too much fuel pressure on your pump, had this problem on a 70 duster, keep fuel cell as close to level with fuel pump as possible, if you have fire backfiring thru carb, you definately have the timing off or cross wired
