I've never seen this part talked about as a backfire fix.  Is there a possibility that it works?

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=35597&item=...

I figured for $15 I'd give it a shot.  As far as I can tell, there is no difference between the valve I received and the stock valve already in the carburetor.  The accompanying spring was ever so slightly different (basically no cadmium plating) but functionally the same.  I noticed no difference whatsoever in engine operation.  I should note that after my rejetting and muffler/air filter replacements, I have very little of the pop on shifting and never on shutdown but that is the same regardless of the air cutoff valve used.  Maybe it helps in a stock bike but I seriously doubt it.  As I said, my visual inspection came up with no difference in the valve.  Bottom line: don't waste your money.