I think its a 1973 440. If I set the initial timing at 10 degrees with the distributor advance vacuum hose disconnected and plugged it often back fires through the carb when I road test it. Seems worse when cold. It runs better at about 20 degrees BTDC but still isn't right. Any ideas what to do to to set it at 10 degrees and tune from there. Its always been like this with different carbs. I have check firing order countless times. Any ideas how to approach this? I dont have any history on the car but the motor still seems to be in decent condition.