Lets deal with this fellow's USA bike, as I doubt he's riding GTR.....
1- GTR had restrictive carb tops that the spring projection moulded in did not allow as much slide opening as US models.
2-many Euro models actually had coolant pasages in the carbs to prevent freezeups.
3-euro models were jetted much leaner
4- I don't believe ALL years were delivered without the air injection
5-I DO believe they were all assembled WITH the air-cut valves on body 1&4
6-Euro bikes were all delivered producing reduced H.P. from the american counterparts.
with that said, we do not know at this time HOW he negated the air injection, and he MAY have a leak, which WOULD cause this problem. Leaky carb boots do not generally create this scenario as that leak is on the inlet side, and intake and exh valves working correctly negate this from occuring. Backfiring (not burble) occurs when a rich exhaust mixture is exposed to oxygen midway or futher from there before the end of the exhaust unless there is an exhaust valve hanging open or timing issue exist. A leaky exhaust pipe gasket is NOT generally a cause, as the pipe is pressurized at this point, and usually does not "suck in air".
The air cut valves still can be dumping excessive raw gas enrichening the mixture as they were intended, and this
can cause the problem as they function only on decelleration mode of the carburation cycle.
We still do not know if the exhaust is stock, or the aftermarket has a non-conducive volume (sized accordingly for the actual flow).
I'm still awaiting Bubba's thought here....