use a metal pipe to reduce the gases coming out of the exhaurst to give it a bit of back presure back ( blocking off the exit hole a bit at a time ) and see what happens,

ive used a socket off a rench before now to give a exhaust back its back preture, ( just use mole grips to hold it in place when testing standing still )

what ever you use make sure it will not come out if you want to run your bike down the road, ( dont want to shoot the car behind you )

its more likley to be the carb fuel mix, but you never know it might help find the fault.