I have a 73 with an L48 350. I installed an Edelbrock intake with a performer carb. The engine runs, but there is an intermittent popping from both tail pipes at idle and there is backfiring through the carb upon acceleration from a standing position although this is somewhat intermittent as well. Here are some of the details respecting set up and some of the things I have tried to solve the problem:
Dwell is at 30 degrees
Timing is at 12 degrees BTDC ( with vacuum hose from distributor plugged)
I get a vacuum reading of 17..the gauge flutters a bit when the engine stumbles at idle, but it doesn't appear to be indicating a valve problem
distributer vacuum hose connected to manifold vacuum port on carb ( I have tried the ported vacuum connection and it doesn't seem to make much difference)
new plug wires...
plugs were new when I installed the intake...I checked them and they are carbon fouled a bit...I cleaned them off and it didn't improve the performance
I have tried turning out the idle mixture screws on the carb ( as many a 4 full turns) since I have read that carb backfire can be a lean condition...but it hasn't made any difference in the backfiring and popping out the exhaust
the backfiring does not happen when I rev the engine in the garage except if I do it when the engine has just fired up...but the popping sound out the tailpipe is always there...
I am at my wits end with this problem...I will admit that I am not a skilled mechanic and I have done most of the work by reading information I get from this forum and other sources, so I am hoping I am just missing something simple due to my "newbie" status.... Any help would be much appreciated!!!

 

Hook up with JustinD ! Just kidding--sounds lean all the way around-check for vacumn leaks--did you have problems before?

 

popping out of the exhaust is afterfiring, out of the carb is backfiring.

Afterfiring is tyical of a too lean condition.

The best way to set your fuel/air mixture is with the vacuum gauge.
hook your gauge up to a full manifold vacuum source port and adjust your mixture so that you get the highest possible vacuum reading. If you have two mixture screws make a slight adjustment on one, let the engine stablize than adjust the other. Make small adjustments at a time.
When you get the best reads, make sure both screws are pretty much the exacly same amount on the settings.

Next double check your timing settings again.
your dwell at 30 is perfect. is it stable when you check it or does it flucuate as you rev up the motor. It should be pretty much rock steady.
You say your timing is at 12* with the vacuum hose unhooked and plugged. That fine for initial timing but what's your total timing and at what point is it all in at? Also, whayt's your total timing with vacuum advance hooked up?
Make sure your total timing w/o vacuum is no more than 36* and that with the vacuum can connected it doesn't exceed 52*.

Once you go through all that and you know the card mixture is correct and your timing is correct, IF you still are having the same problem that you may have to start looking for a vacuum leak somewhere. Vacuum leaks cause a lean condition.

oh, just thought of something else....... do you still have youe A.I.R system installed? Another common cause of afterfiring on decel on cars with A.I.R. pumps is a ruptured diaphragm in the mixture control valve (early cars) or in the diverter valve (later cars). When that diaphragm fails, the valve defaults to the open position, and air from the pump is injected 100% of the time, causing decel afterfiring; in addition, the diaphragm failure also induces a massive vacuum leak through the signal line, which leans out the mixture and retards idle and cruise timing.

 

Thanks for the advice...the dwell is not rock solid even at idle..it jumps around a bit...by a few 10ths...it also changes when I rev the motor. I am not sure what an AIR system is..the original manifold was EGR but I replaced it with a non EGR edelbrock...hope that isn't part of my problem. I tried adjusting the air/fuel ratio using my vacuum gauge but it did not solve the backfire problem. The backfire seems to happen as I start to accelerate...once I am going it will cruise along o.k. although there doesn't seem to be much pep to the engine.

 

Vacuum leak will cause that as well,..are you sure about all the hoses, gaskets, etc?

Sometimes you can hear the leak.