Popping is basically caused by a leak in the exhaust system possibly at the head pipe or at a joint in the system that draws in air. When any unburnt fuel exits the cylinder like on decel, the air that can leak into the system will cause the mixture in the system to ignite causing a pop. Check your system for leaks and repair as necessary. To locate any leaks you may have at the joints in the exhaust or at the mufflers, try starting the bike and with some heavy gloves on, block the exhaust at the outlets and listen for a hissing. You can locate any leaks this way. It may be better if you have a helper listen for the hissing while you block the exit of the exhaust from the mufflers.