I don't know how much tuning you have tried to do, but if you reset it back to factory settings and you still have the same problem, you might need to clean your carburetor.
Both a friend and I had the same issue with our Revos and after a clean of the carburetor it went away and things have been sweet.
It might also be an advantage to run an inline fuel filter. That might stop some dirt and crap getting in there in the first place (I am not running one yet - but I really should be getting one ASAP!).
I have heard that you may need to adjust HSN and LSN settings a little richer when running a new or different pipe, but I would have thought that factory settings would be rich enough to be safe all the same. I'll let someone else with more experience on aftermarket pipes confirm that one though.
-Brad