Classic/Antique Car Repair: 74 nova follow-up, vacuum leak, vacuum line

Question
Ok,  I adjusted distributor 180 degrees because it was off like you said and that took care of carb backfire.  Cranks right up now and sounds smooth with the exception of the passenger side backfiring from tailpipe.  If its not one thing its another!  So, lets see what would be the easiest, cheapest, and most likely thing to cause this?  I hope we can go in that order but with my luck and this car who knows.  On a serious note I would appreciate a list of ideas of things to check.  I have read some things that mentioned valve springs, vacuum line leaks, carb mount leaks, and as simple as higher grade fuel could cure my sickness. What do you think?

Answer
OK, tell me more about this backfire.  Also, tell me how the engine feels otherwise - does the power seem normal - does it sound OK? How's the idle?  What is the situation with the moisture and smoke out the tailpipes?

Does the backfire occur on every revolution of the engine, or just randomly, once in a while?  When it backfires, what does it sound like - a firecracker going off, or just a loud "chuff" like  steam engine sound out the pipe?   Do you have the backfire at idle, or only when you are driving down the street - if when driving, does it go away when you are accelerating and only backfire when you let off the gas, or vice-versa.

Reason for all these questions is that I am trying to figure out if this is a malfunction of your smog system (do you have all the factory stuff on the engine - for example the AIR pump (air injection reactor) and all the plumbing and valves that came with it. It could also be a vacuum leak - but if that were the case, it would idle poorly.

The way to check for a vacuum leak is to let the engine idle, with the air cleaner off the car, and slowly slide a piece of something non-combustible across the top of the carburetor.  I do it with my hand, but I can't tell anyone to do that because if it spits back at you, you might get singed, and I don't want to get sued - so find a piece of metal and slide this across the top of the carburetor.  If at ANY point as you block off more and more of the air flow, the engine actually SPEEDS UP, you have a vacuum leak.   Write back with some more info for me, and we'll go the next mile on this.

Dick