<VV> exhaust afterfire "BANG"

[Michael Kovacs]

I'm looking for advice on an after fire. The 140 "pops" about every 10-20 seconds after being thoroughly warmed up and just idling. I'm assuming it is an rich condition that allow a too rich mixture to build up in the muffler and then bang when conditions are ripe. 
   
   The choke on the primary carb is fully open and the secondary is drawing no air that can be measured with a unisync.
   
   Possible cause? The idle screw does not seem to have any effect either backed out 4-5 turns or lightly against the seat. This seems to be a new problem. The car gets very little use and only has 5,000 miles in the last 25 years, but the carbs were rebuilt 3 years ago.
   
   I just changed out the carb off one other operational 140 ,but haven't been able to drive it (to really warm it up) due to the slushy road conditions. 
   
   All analysis accepted.