I think I found the solution to the popping today!!
I have a '97 Beta Techno I have desperately been trying to quiet down so I can ride around my local mountain trails here in San Diego.
I put in a new plug, gapped it as suggested, and re-packed the silencer 5 ways, no joy.
Finally took someone's suggestion to lean out the bike with the pilot screw. The pilot is on the carb on the right side of the bike. It is to the left of the pilot air screw that adjusts the idle using your fingers.
Backing the pilot screw (not the idle screw) out (counter-clockwise) lets in more AIR (not fuel, as I had supposed). More air makes the bike lean. When the bike is running rich (too much gas) it causes the popping.
Here's how someone suggested you set it. Get the bike fully warm - actually go ride it around. Get off the bike with it idling. Turn the pilot screw - the one you have to use a screwdriver for - all the way in (clockwise).
Next back the screw out somewhat slowly and listen to for the idle to speed up. I think mine backed out two or three turns from all the way in. More than this and you need a new pilot jet, apparently. When the idle speed is no longer increasing, that's where you are supposed to stop.
Your bike may be idling a little faster now, so adjust accordingly with the idle control (you use your fingers on this one).
Incredibly my bike no longer pops at idle (at least while hot - have not tested from cold start up). I'm thrilled. Even coming back down from half throttle - no pops.
Another tip - if you really need to quiet the bike down to get to a riding spot past some houses, take an old grip (dirt bike, throttle side as it has a larger diameter). Cut a 1" slit in the grip from mid way back to the end of the grip. Start the bike up then put the grip on, the slit should point down to the ground.
This will not cause any problems as far a keeping the bike in the lower revs to get to where you need to be. It has never over heated any of my bikes and I've done this on two and four strokes. Once you get to your spot, pull the grip off and stick it in your pocket. It makes a remarkable difference to exhaust volume. Sorry for the long post!