1973 340 Duster Resto

Having a backfiring problem at the moment. With both the Eddy I had on it and the spreadbore. Gettin frustration all wire are on correctly same way as i took them off when new cap was on. I can get it to run if I keep my foot on the gas but if I try to let it Idle it backfires through the carb and dies. Its gettin plenty of gas.
Would a vacuum leak cause it to back fire?