'...cheap emgo pods are, well, exactly that, cheap. K&N pods are like night and day!'
Really. I've run them both and both make same noise and work equally well. I have run the foam UNI as well and they work great too. I currently use APE which are K&N clones at 1/2 price, I loathe how K&N got famous and tripled prices 5 minutes later. They certainly run no better and I for one refuse to get ripped off by their incredibly high prices.
Backfire is commonly a loaded up plug to foul spark down the side of it. A weak ignition say bad plug wire or connection can make a plug look just like it it running rich. Are all plug gaps the same and to spec?
Like said, so many buy bike that has dead motor in some way (they ARE 40 years old after all!) and then pull hair out trying to find the issue when it runs like crap. ALWAYS start with a known good engine, the compression has to be a good running number (putting your finger over the hole as a test shows how incompetent you are there) before going anywhere else.
So don't listen...............you WILL eventually.