lean condition will give similar behavior of afterfire (backfire is through the carb, afterfire is in the exhaust) and/or a hanging idle. For comparison, an idle that stumbles below idle rpm on throttle blip is usually a rich condition.
It's hard to use the popping as a diagnostic tool, as both lean and rich can cause popping. I find it's usually a lean condition if it pops, as rich usually means it's too rich to actually fire the mixture and when it does, it's either weak or a heck of a loud bang, not a pop. Lean causes afterfire because it's too lean to burn completely in the cylinder, so when the exhaust valve opens unburnt (but otherwise combustible) mixture enters the exhaust and then combusts in the exhaust, causing the "pop" noise.
I've used the choke before to help diagnose rich vs lean. If it runs better on choke (with engine at normal operating temp), you're lean. Worse with choke, you're rich. I take the bike for a short run and if it's popping, I slowly feed on the choke. If it goes away, I can look for a lean fueling condition. If I can't make it run any better or it gets worse with choke, I can look for a rich fueling condition. It's a "feel" you need to develop through experience as it can be hard to tell if an engine starts running better or worse as you play with the choke. It's a quick and dirty way to give you an indication of which way to go with out having to do things like plug chops. But it's just an indicator, not a tuning tool for final jetting. For that, you have to go through the full jetting procedure.
lean condition can either be jetting or idle/air screws or it can be a vacuum leak after carbs, assuming the rest of the bike is in good nick and running properly (IE: timing, coils, compression, etc).
If it's all stock, that pretty much eliminates jet sizing as an issue.
Check the idle mixture screws next and set as per the manual.
Vacuum leak is usually a cracked intake rubber or leak in that area. I often use something like WD40 or a non combustible spray on the runners to see if the idle changes. Generally, if you spray the rubbers and the idle changes, you've got an intake leak somewhere. The idle changes (usually drops) as the spray closes up the leak as it's sucked thorough and changes again as it's fulled sucked out of any cracks or splits and goes back to being an air leak.
