Well, exhaust systems aren't closed systems. There is always ambient flowing around air inside the muffler. For example, if you look at the bottom of the muffler, you can see little holes which allow trapped moisture to escape and which also let outside air in.

As far as what you were describing, slight popping and gurgling sounds aren't considered afterfire. And I have experienced then also when in a lean mixture.
Afterfire is the serious bang that everyone has heard at some point in their lives, but it comes out of the exhaust.

Like I said, it is mistakenly referred to as backfire.

I'm not a chemist or physics expert, so it's possible that afterfiring can occur in a lean mixture, I just don't see how it could happen. When there is too much air in cylinder, an engine doesn't experience a proper explosion. It would make sense to me that a lean mixture which carries over into a muffler would then encounter even more air, which would essentially kill the flame.

I may be wrong, but I'm pretty sure that's how it works.

choke
 

Hey Easy thanks for the correction I went on-line and read about cv carbs . The word " choke " is a legacy term as you pointed out the " enricher " circuit provides a richer mixture. I did some testing in late Oct and early Nov . In cold weather ( 40 F ) I applied a little " choke " when the top speed maxed @ 65 approx 6500 rpm and top went to 70 and 7000 rpm. It dropped back to 6500 rpm after opening " choke " again.

This is just an observation .

I have a tach .

Re: choke
 

Gotta give that a try.
I've always heard that the "enricher" was out of the picture at high load/rpm.
Since it doesn't restrict the input air flow like a real choke, I can imagine that it might still make the mixture a tiny bit richer without "choking" it.