Just cleaned that air filter, and it certainly needed it.
Did the .22 cent mod to the carb, but it didn't "fix" the pop/backfire on deceleration. instead, it just changed the popping frequency and timbre. prior to the mod, the pops were quick in succession, sometimes followed by a loud pop. now, it's about 2 or 3 muted pops, followed by a loud, low frequency pop. the mod did something, but it didn't fix it. perhaps a valve check is in order. I also noticed that the choke plunger cap was missing a little piece. looks like a previous owner chipped the piece off trying to mess with the carb. i'll have to switch that out, but i doubt that would settle the popping issue.
i second-guessed my work, and took everything apart again to make sure everything was tight. everything was exactly as it should be.
the GOOD news is that i've got my throttle back, which is nice.
Ever known anyone to run into the same problem after doing the .22 cent mod?