I agree with Matt. Sometimes you have to remove the mufflers and install them to the headers with high temp silicone sealant. You might want to make sure the headers are tight to the head (don't break the studs though). Opening the idle mixture (pilot screws) a bit also helps with popping. The best thing I did was to disconnect my throttle position sensor. Make sure you unplug it only and not remove it from the bike. Many of us have done this for years with no problems but the popping all but disappears.
