1961 Questions for all other cars 1958 edsel backfires

I have a 1958 edsel that starts and runs in idle. When the car runs in idle the cars runs slugish. The car has true dual exhaust on the passenger side pipe the cars starts to burp then backfires and only gets louder the longer it runs and when it back fires it looks like an a bomb is coming out the back pipe I am afraid of running it for long time becasue of the loud noise. I have repled all vacume lines and no leaks can be found I do not know what to do now or what steps to take in fixing this problem any suggestions or help would be great.