Blocking Pair Valve

[Batfink]

The PAIR valve inject's clean air into the exhaust system to "help" with emissions.. all it really does is dilute the emissions with fresh air LOL

If you put on an aftermarket can(s) then you may be upsetting this equation. Most after market cans are less restrictive than the OEM cans which allows more unburnt fuel to enter exhaust pipes. As soon as this fuel hits the fresh air injected by the PAIR valve the mixture allows an further explosion (backfire). This will happen more from backing off the throttle, specially after a big blast.

To fix this you can either restrict the cans more by using a baffle, detune the bike to have less unburnt fuel or remove the injection of clean air by blocking off the PAIR valve.
Other than adding a baffle, you will need to get bike on some type of tuning software that can manipulate the ECU. If you just block the PAIR valve you WILL get a fault code as the sensor will be getting a wrong reading, flashing can remove the need for this sensor thus not bring up a fault code.

I hope I have this right LOL