I would like to preface this with -- I really don't know s#!t.
I bought a XR70 for my second son and this weekend he was out riding when my dad showed up. (Engine Master - trust me, I know not what the hell he is talking about, without a considerable amount of research)
Anyway, the XR70 was "popping" as well and I thought that is was probably running rich, in accordance with previous replies.
The 'old man' told me to check to make sure that the exhaust bolts were tight, as that is a typical sign. I wouldn't say that they were loose, but I did tighten them more (too tight?). The popping didn't stop but I can't say that it didn't help either. I became concerned about other things like, 'If you want to go faster, SHIFT!'
Just thought that I would pass it along that it might be just leaky exhaust.