Backfiring after re-installing smog pump

[notrod59]

They changed the emission laws where I live, so I had to put the smog pump back on my 81 Blazer. I finally got it going, after 5 trips to NAPA to get the right belt, but now it's backfiring when I take my foot off the gas pedal. To my knowledge the only thing the smog pump does is inject air into the headers and dilute the exhaust (giving the illusion of cleaner emissions). I'm not sure why this would make a difference. I didn't mess with the timing or carb mixture or do I need to? I think I had the same problem 10+ years ago when I had to have emissions but I can't remember. Any suggestions?